🏗 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.h

@@ -0,0 +1,52 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/core/helpers.h"
+
+namespace esphome {
+namespace text_sensor {
+
+#define LOG_TEXT_SENSOR(prefix, type, obj) \
+  if (obj != nullptr) { \
+    ESP_LOGCONFIG(TAG, prefix type " '%s'", obj->get_name().c_str()); \
+    if (!obj->get_icon().empty()) { \
+      ESP_LOGCONFIG(TAG, prefix "  Icon: '%s'", obj->get_icon().c_str()); \
+    } \
+    if (!obj->unique_id().empty()) { \
+      ESP_LOGV(TAG, prefix "  Unique ID: '%s'", obj->unique_id().c_str()); \
+    } \
+  }
+
+class TextSensor : public Nameable {
+ public:
+  explicit TextSensor(const std::string &name);
+  explicit TextSensor();
+
+  void publish_state(std::string state);
+
+  void set_icon(const std::string &icon);
+
+  void add_on_state_callback(std::function<void(std::string)> callback);
+
+  std::string state;
+
+  // ========== INTERNAL METHODS ==========
+  // (In most use cases you won't need these)
+  std::string get_icon();
+
+  virtual std::string icon();
+
+  virtual std::string unique_id();
+
+  bool has_state();
+
+ protected:
+  uint32_t hash_base() override;
+
+  CallbackManager<void(std::string)> callback_;
+  optional<std::string> icon_;
+  bool has_state_{false};
+};
+
+}  // namespace text_sensor
+}  // namespace esphome