🏗 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/core/preferences.h

@@ -0,0 +1,92 @@
+#pragma once
+
+#include <string>
+
+#ifdef ARDUINO_ARCH_ESP32
+#include <Preferences.h>
+#endif
+
+#include "esphome/core/esphal.h"
+
+namespace esphome {
+
+class ESPPreferenceObject {
+ public:
+  ESPPreferenceObject();
+  ESPPreferenceObject(size_t rtc_offset, size_t length, uint32_t type);
+
+  template<typename T> bool save(T *src);
+
+  template<typename T> bool load(T *dest);
+
+  bool is_initialized() const;
+
+ protected:
+  bool save_();
+  bool load_();
+  bool save_internal_();
+  bool load_internal_();
+
+  uint32_t calculate_crc_() const;
+
+  size_t rtc_offset_;
+  size_t length_words_;
+  uint32_t type_;
+  uint32_t *data_;
+};
+
+class ESPPreferences {
+ public:
+  ESPPreferences();
+  void begin(const std::string &name);
+  ESPPreferenceObject make_preference(size_t length, uint32_t type);
+  template<typename T> ESPPreferenceObject make_preference(uint32_t type);
+
+#ifdef ARDUINO_ARCH_ESP8266
+  /** On the ESP8266, we can't override the first 128 bytes during OTA uploads
+   * as the eboot parameters are stored there. Writing there during an OTA upload
+   * would invalidate applying the new firmware. During normal operation, we use
+   * this part of the RTC user memory, but stop writing to it during OTA uploads.
+   *
+   * @param prevent Whether to prevent writing to the first 32 words of RTC user memory.
+   */
+  void prevent_write(bool prevent);
+  bool is_prevent_write();
+#endif
+
+ protected:
+  friend ESPPreferenceObject;
+
+  uint32_t current_offset_;
+#ifdef ARDUINO_ARCH_ESP32
+  Preferences preferences_;
+#endif
+#ifdef ARDUINO_ARCH_ESP8266
+  bool prevent_write_{false};
+#endif
+};
+
+extern ESPPreferences global_preferences;
+
+template<typename T> ESPPreferenceObject ESPPreferences::make_preference(uint32_t type) {
+  return this->make_preference((sizeof(T) + 3) / 4, type);
+}
+
+template<typename T> bool ESPPreferenceObject::save(T *src) {
+  if (!this->is_initialized())
+    return false;
+  memset(this->data_, 0, this->length_words_ * 4);
+  memcpy(this->data_, src, sizeof(T));
+  return this->save_();
+}
+
+template<typename T> bool ESPPreferenceObject::load(T *dest) {
+  memset(this->data_, 0, this->length_words_ * 4);
+  if (!this->load_())
+    return false;
+
+  memcpy(dest, this->data_, sizeof(T));
+  return true;
+}
+
+}  // namespace esphome