🏗 Merge C++ into python codebase (#504)

## Description:

Move esphome-core codebase into esphome (and a bunch of other refactors). See https://github.com/esphome/feature-requests/issues/97

Yes this is a shit ton of work and no there's no way to automate it :( But it will be worth it 👍

Progress:
- Core support (file copy etc): 80%
- Base Abstractions (light, switch): ~50%
- Integrations: ~10%
- Working? Yes, (but only with ported components).

Other refactors:
- Moves all codegen related stuff into a single class: `esphome.codegen` (imported as `cg`)
- Rework coroutine syntax
- Move from `component/platform.py` to `domain/component.py` structure as with HA
- Move all defaults out of C++ and into config validation.
- Remove `make_...` helpers from Application class. Reason: Merge conflicts with every single new integration.
- Pointer Variables are stored globally instead of locally in setup(). Reason: stack size limit.

Future work:
- Rework const.py - Move all `CONF_...` into a conf class (usage `conf.UPDATE_INTERVAL` vs `CONF_UPDATE_INTERVAL`). Reason: Less convoluted import block
- Enable loading from `custom_components` folder.

**Related issue (if applicable):** https://github.com/esphome/feature-requests/issues/97

**Pull request in [esphome-docs](https://github.com/esphome/esphome-docs) with documentation (if applicable):** esphome/esphome-docs#<esphome-docs PR number goes here>

## Checklist:
  - [ ] The code change is tested and works locally.
  - [ ] Tests have been added to verify that the new code works (under `tests/` folder).

If user exposed functionality or configuration variables are added/changed:
  - [ ] Documentation added/updated in [esphomedocs](https://github.com/OttoWinter/esphomedocs).

esphome/components/htu21d/htu21d.cpp

@@ -0,0 +1,63 @@
+#include "htu21d.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace htu21d {
+
+static const char *TAG = "htu21d";
+
+static const uint8_t HTU21D_ADDRESS = 0x40;
+static const uint8_t HTU21D_REGISTER_RESET = 0xFE;
+static const uint8_t HTU21D_REGISTER_TEMPERATURE = 0xE3;
+static const uint8_t HTU21D_REGISTER_HUMIDITY = 0xE5;
+static const uint8_t HTU21D_REGISTER_STATUS = 0xE7;
+
+void HTU21DComponent::setup() {
+  ESP_LOGCONFIG(TAG, "Setting up HTU21D...");
+
+  if (!this->write_bytes(HTU21D_REGISTER_RESET, nullptr, 0)) {
+    this->mark_failed();
+    return;
+  }
+
+  // Wait for software reset to complete
+  delay(15);
+}
+void HTU21DComponent::dump_config() {
+  ESP_LOGCONFIG(TAG, "HTU21D:");
+  LOG_I2C_DEVICE(this);
+  if (this->is_failed()) {
+    ESP_LOGE(TAG, "Communication with HTU21D failed!");
+  }
+  LOG_UPDATE_INTERVAL(this);
+  LOG_SENSOR("  ", "Temperature", this->temperature_);
+  LOG_SENSOR("  ", "Humidity", this->humidity_);
+}
+void HTU21DComponent::update() {
+  uint16_t raw_temperature;
+  if (!this->read_byte_16(HTU21D_REGISTER_TEMPERATURE, &raw_temperature, 50)) {
+    this->status_set_warning();
+    return;
+  }
+
+  float temperature = (float(raw_temperature & 0xFFFC)) * 175.72f / 65536.0f - 46.85f;
+
+  uint16_t raw_humidity;
+  if (!this->read_byte_16(HTU21D_REGISTER_HUMIDITY, &raw_humidity, 50)) {
+    this->status_set_warning();
+    return;
+  }
+
+  float humidity = (float(raw_humidity & 0xFFFC)) * 125.0f / 65536.0f - 6.0f;
+  ESP_LOGD(TAG, "Got Temperature=%.1f°C Humidity=%.1f%%", temperature, humidity);
+
+  if (this->temperature_ != nullptr)
+    this->temperature_->publish_state(temperature);
+  if (this->humidity_ != nullptr)
+    this->humidity_->publish_state(humidity);
+  this->status_clear_warning();
+}
+float HTU21DComponent::get_setup_priority() const { return setup_priority::DATA; }
+
+}  // namespace htu21d
+}  // namespace esphome

esphome/components/htu21d/htu21d.h

@@ -0,0 +1,30 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/components/sensor/sensor.h"
+#include "esphome/components/i2c/i2c.h"
+
+namespace esphome {
+namespace htu21d {
+
+class HTU21DComponent : public PollingComponent, public i2c::I2CDevice {
+ public:
+  HTU21DComponent(uint32_t update_interval) : PollingComponent(update_interval) {}
+  void set_temperature(sensor::Sensor *temperature) { temperature_ = temperature; }
+  void set_humidity(sensor::Sensor *humidity) { humidity_ = humidity; }
+
+  /// Setup (reset) the sensor and check connection.
+  void setup() override;
+  void dump_config() override;
+  /// Update the sensor values (temperature+humidity).
+  void update() override;
+
+  float get_setup_priority() const override;
+
+ protected:
+  sensor::Sensor *temperature_{nullptr};
+  sensor::Sensor *humidity_{nullptr};
+};
+
+}  // namespace htu21d
+}  // namespace esphome

esphome/components/htu21d/sensor.py

@@ -0,0 +1,33 @@
+import esphome.codegen as cg
+import esphome.config_validation as cv
+from esphome.components import i2c, sensor
+from esphome.const import CONF_HUMIDITY, CONF_ID, CONF_TEMPERATURE, \
+    CONF_UPDATE_INTERVAL, ICON_THERMOMETER, UNIT_CELSIUS, UNIT_PERCENT, ICON_WATER_PERCENT
+
+DEPENDENCIES = ['i2c']
+
+htu21d_ns = cg.esphome_ns.namespace('htu21d')
+HTU21DComponent = htu21d_ns.class_('HTU21DComponent', cg.PollingComponent, i2c.I2CDevice)
+
+CONFIG_SCHEMA = cv.Schema({
+    cv.GenerateID(): cv.declare_variable_id(HTU21DComponent),
+    cv.Required(CONF_TEMPERATURE): cv.nameable(
+        sensor.sensor_schema(UNIT_CELSIUS, ICON_THERMOMETER, 1)),
+    cv.Required(CONF_HUMIDITY): cv.nameable(
+        sensor.sensor_schema(UNIT_PERCENT, ICON_WATER_PERCENT, 1)),
+    cv.Optional(CONF_UPDATE_INTERVAL, default='60s'): cv.update_interval,
+}).extend(cv.COMPONENT_SCHEMA).extend(i2c.i2c_device_schema(0x40))
+
+
+def to_code(config):
+    var = cg.new_Pvariable(config[CONF_ID], config[CONF_UPDATE_INTERVAL])
+    yield cg.register_component(var, config)
+    yield i2c.register_i2c_device(var, config)
+
+    if CONF_TEMPERATURE in config:
+        sens = yield sensor.new_sensor(config[CONF_TEMPERATURE])
+        cg.add(var.set_temperature(sens))
+
+    if CONF_HUMIDITY in config:
+        sens = yield sensor.new_sensor(config[CONF_HUMIDITY])
+        cg.add(var.set_humidity(sens))