🏗 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/api/subscribe_state.cpp

@@ -0,0 +1,77 @@
+#include "subscribe_state.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace api {
+
+#ifdef USE_BINARY_SENSOR
+bool InitialStateIterator::on_binary_sensor(binary_sensor::BinarySensor *binary_sensor) {
+  if (!binary_sensor->has_state())
+    return true;
+
+  return this->client_->send_binary_sensor_state(binary_sensor, binary_sensor->state);
+}
+#endif
+#ifdef USE_COVER
+bool InitialStateIterator::on_cover(cover::Cover *cover) { return this->client_->send_cover_state(cover); }
+#endif
+#ifdef USE_FAN
+bool InitialStateIterator::on_fan(fan::FanState *fan) { return this->client_->send_fan_state(fan); }
+#endif
+#ifdef USE_LIGHT
+bool InitialStateIterator::on_light(light::LightState *light) { return this->client_->send_light_state(light); }
+#endif
+#ifdef USE_SENSOR
+bool InitialStateIterator::on_sensor(sensor::Sensor *sensor) {
+  if (!sensor->has_state())
+    return true;
+
+  return this->client_->send_sensor_state(sensor, sensor->state);
+}
+#endif
+#ifdef USE_SWITCH
+bool InitialStateIterator::on_switch(switch_::Switch *a_switch) {
+  return this->client_->send_switch_state(a_switch, a_switch->state);
+}
+#endif
+#ifdef USE_TEXT_SENSOR
+bool InitialStateIterator::on_text_sensor(text_sensor::TextSensor *text_sensor) {
+  if (!text_sensor->has_state())
+    return true;
+
+  return this->client_->send_text_sensor_state(text_sensor, text_sensor->state);
+}
+#endif
+#ifdef USE_CLIMATE
+bool InitialStateIterator::on_climate(climate::Climate *climate) { return this->client_->send_climate_state(climate); }
+#endif
+InitialStateIterator::InitialStateIterator(APIServer *server, APIConnection *client)
+    : ComponentIterator(server), client_(client) {}
+
+APIMessageType SubscribeStatesRequest::message_type() const { return APIMessageType::SUBSCRIBE_STATES_REQUEST; }
+
+bool HomeAssistantStateResponse::decode_length_delimited(uint32_t field_id, const uint8_t *value, size_t len) {
+  switch (field_id) {
+    case 1:
+      // string entity_id = 1;
+      this->entity_id_ = as_string(value, len);
+      return true;
+    case 2:
+      // string state = 2;
+      this->state_ = as_string(value, len);
+      return true;
+    default:
+      return false;
+  }
+}
+APIMessageType HomeAssistantStateResponse::message_type() const {
+  return APIMessageType::HOME_ASSISTANT_STATE_RESPONSE;
+}
+const std::string &HomeAssistantStateResponse::get_entity_id() const { return this->entity_id_; }
+const std::string &HomeAssistantStateResponse::get_state() const { return this->state_; }
+APIMessageType SubscribeHomeAssistantStatesRequest::message_type() const {
+  return APIMessageType::SUBSCRIBE_HOME_ASSISTANT_STATES_REQUEST;
+}
+
+}  // namespace api
+}  // namespace esphome