🏗 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/mqtt/mqtt_cover.h

@@ -0,0 +1,42 @@
+#pragma once
+
+#include "esphome/core/defines.h"
+#include "mqtt_component.h"
+
+#ifdef USE_COVER
+
+#include "esphome/components/cover/cover.h"
+
+namespace esphome {
+namespace mqtt {
+
+class MQTTCoverComponent : public mqtt::MQTTComponent {
+ public:
+  explicit MQTTCoverComponent(cover::Cover *cover);
+
+  void setup() override;
+  void send_discovery(JsonObject &root, mqtt::SendDiscoveryConfig &config) override;
+
+  MQTT_COMPONENT_CUSTOM_TOPIC(position, command)
+  MQTT_COMPONENT_CUSTOM_TOPIC(position, state)
+  MQTT_COMPONENT_CUSTOM_TOPIC(tilt, command)
+  MQTT_COMPONENT_CUSTOM_TOPIC(tilt, state)
+
+  bool send_initial_state() override;
+  bool is_internal() override;
+
+  bool publish_state();
+
+  void dump_config() override;
+
+ protected:
+  std::string component_type() const override;
+  std::string friendly_name() const override;
+
+  cover::Cover *cover_;
+};
+
+}  // namespace mqtt
+}  // namespace esphome
+
+#endif