🏗 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/homeassistant/sensor/__init__.py

@@ -0,0 +1,20 @@
+import esphome.codegen as cg
+import esphome.config_validation as cv
+from esphome.components import sensor
+from esphome.const import CONF_ENTITY_ID, CONF_ID, CONF_NAME
+from .. import homeassistant_ns
+
+DEPENDENCIES = ['api']
+
+HomeassistantSensor = homeassistant_ns.class_('HomeassistantSensor', sensor.Sensor)
+
+CONFIG_SCHEMA = cv.nameable(sensor.SENSOR_SCHEMA.extend({
+    cv.GenerateID(): cv.declare_variable_id(HomeassistantSensor),
+    cv.Required(CONF_ENTITY_ID): cv.entity_id,
+}))
+
+
+def to_code(config):
+    var = cg.new_Pvariable(config[CONF_ID], config[CONF_NAME], config[CONF_ENTITY_ID])
+    yield cg.register_component(var, config)
+    yield sensor.register_sensor(var, config)

esphome/components/homeassistant/sensor/homeassistant_sensor.cpp

@@ -0,0 +1,32 @@
+#include "homeassistant_sensor.h"
+#include "esphome/core/log.h"
+#include "esphome/components/api/api_server.h"
+
+namespace esphome {
+namespace homeassistant {
+
+static const char *TAG = "homeassistant.sensor";
+
+HomeassistantSensor::HomeassistantSensor(const std::string &name, const std::string &entity_id)
+    : Sensor(name), entity_id_(entity_id) {}
+void HomeassistantSensor::setup() {
+  api::global_api_server->subscribe_home_assistant_state(this->entity_id_, [this](std::string state) {
+    auto val = parse_float(state);
+    if (!val.has_value()) {
+      ESP_LOGW(TAG, "Can't convert '%s' to number!", state.c_str());
+      this->publish_state(NAN);
+      return;
+    }
+
+    ESP_LOGD(TAG, "'%s': Got state %.2f", this->entity_id_.c_str(), *val);
+    this->publish_state(*val);
+  });
+}
+void HomeassistantSensor::dump_config() {
+  LOG_SENSOR("", "Homeassistant Sensor", this);
+  ESP_LOGCONFIG(TAG, "  Entity ID: '%s'", this->entity_id_.c_str());
+}
+float HomeassistantSensor::get_setup_priority() const { return setup_priority::AFTER_CONNECTION; }
+
+}  // namespace homeassistant
+}  // namespace esphome

esphome/components/homeassistant/sensor/homeassistant_sensor.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/components/sensor/sensor.h"
+
+namespace esphome {
+namespace homeassistant {
+
+class HomeassistantSensor : public sensor::Sensor, public Component {
+ public:
+  HomeassistantSensor(const std::string &name, const std::string &entity_id);
+  void setup() override;
+  void dump_config() override;
+  float get_setup_priority() const override;
+
+ protected:
+  std::string entity_id_;
+};
+
+}  // namespace homeassistant
+}  // namespace esphome