🏗 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/text_sensor/text_sensor.cpp

@@ -0,0 +1,33 @@
+#include "text_sensor.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace text_sensor {
+
+static const char *TAG = "text_sensor";
+
+TextSensor::TextSensor() : TextSensor("") {}
+TextSensor::TextSensor(const std::string &name) : Nameable(name) {}
+
+void TextSensor::publish_state(std::string state) {
+  this->state = state;
+  this->has_state_ = true;
+  ESP_LOGD(TAG, "'%s': Sending state '%s'", this->name_.c_str(), state.c_str());
+  this->callback_.call(state);
+}
+void TextSensor::set_icon(const std::string &icon) { this->icon_ = icon; }
+void TextSensor::add_on_state_callback(std::function<void(std::string)> callback) {
+  this->callback_.add(std::move(callback));
+}
+std::string TextSensor::get_icon() {
+  if (this->icon_.has_value())
+    return *this->icon_;
+  return this->icon();
+}
+std::string TextSensor::icon() { return ""; }
+std::string TextSensor::unique_id() { return ""; }
+bool TextSensor::has_state() { return this->has_state_; }
+uint32_t TextSensor::hash_base() { return 334300109UL; }
+
+}  // namespace text_sensor
+}  // namespace esphome