🏗 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/wifi_signal/sensor.py

@@ -0,0 +1,24 @@
+import esphome.codegen as cg
+import esphome.config_validation as cv
+from esphome.components import sensor
+from esphome.const import CONF_ID, CONF_NAME, CONF_UPDATE_INTERVAL, CONF_ICON, \
+    CONF_UNIT_OF_MEASUREMENT, CONF_ACCURACY_DECIMALS, ICON_WIFI, UNIT_DECIBEL
+
+DEPENDENCIES = ['wifi']
+wifi_signal_ns = cg.esphome_ns.namespace('wifi_signal')
+WiFiSignalSensor = wifi_signal_ns.class_('WiFiSignalSensor', sensor.PollingSensorComponent)
+
+CONFIG_SCHEMA = cv.nameable(sensor.SENSOR_SCHEMA.extend({
+    cv.GenerateID(): cv.declare_variable_id(WiFiSignalSensor),
+    cv.Optional(CONF_UPDATE_INTERVAL, default='60s'): cv.update_interval,
+
+    cv.Optional(CONF_ICON, default=ICON_WIFI): sensor.icon,
+    cv.Optional(CONF_UNIT_OF_MEASUREMENT, default=UNIT_DECIBEL): sensor.unit_of_measurement,
+    cv.Optional(CONF_ACCURACY_DECIMALS, default=0): sensor.accuracy_decimals,
+}).extend(cv.COMPONENT_SCHEMA))
+
+
+def to_code(config):
+    var = cg.new_Pvariable(config[CONF_ID], config[CONF_NAME], config[CONF_UPDATE_INTERVAL])
+    yield cg.register_component(var, config)
+    yield sensor.register_sensor(var, config)

esphome/components/wifi_signal/wifi_signal_sensor.cpp

@@ -0,0 +1,12 @@
+#include "wifi_signal_sensor.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace wifi_signal {
+
+static const char *TAG = "wifi_signal.sensor";
+
+void WiFiSignalSensor::dump_config() { LOG_SENSOR("", "WiFi Signal", this); }
+
+}  // namespace wifi_signal
+}  // namespace esphome

esphome/components/wifi_signal/wifi_signal_sensor.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/core/helpers.h"
+#include "esphome/components/sensor/sensor.h"
+#include "esphome/components/wifi/wifi_component.h"
+
+namespace esphome {
+namespace wifi_signal {
+
+class WiFiSignalSensor : public sensor::PollingSensorComponent {
+ public:
+  explicit WiFiSignalSensor(const std::string &name, uint32_t update_interval)
+      : sensor::PollingSensorComponent(name, update_interval) {}
+
+  void update() override { this->publish_state(WiFi.RSSI()); }
+  void dump_config() override;
+
+  std::string unique_id() override { return get_mac_address() + "-wifisignal"; }
+  float get_setup_priority() const override { return setup_priority::AFTER_WIFI; }
+};
+
+}  // namespace wifi_signal
+}  // namespace esphome