🏗 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/adc/adc_sensor.h

@@ -0,0 +1,43 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/core/esphal.h"
+#include "esphome/components/sensor/sensor.h"
+
+namespace esphome {
+namespace adc {
+
+class ADCSensor : public sensor::PollingSensorComponent {
+ public:
+  /// Construct the ADCSensor with the provided pin and update interval in ms.
+  explicit ADCSensor(const std::string &name, uint8_t pin, uint32_t update_interval);
+
+#ifdef ARDUINO_ARCH_ESP32
+  /// Set the attenuation for this pin. Only available on the ESP32.
+  void set_attenuation(adc_attenuation_t attenuation);
+#endif
+
+  // ========== INTERNAL METHODS ==========
+  // (In most use cases you won't need these)
+  /// Update adc values.
+  void update() override;
+  /// Setup ADc
+  void setup() override;
+  void dump_config() override;
+  /// `HARDWARE_LATE` setup priority.
+  float get_setup_priority() const override;
+
+#ifdef ARDUINO_ARCH_ESP8266
+  std::string unique_id() override;
+#endif
+
+ protected:
+  uint8_t pin_;
+
+#ifdef ARDUINO_ARCH_ESP32
+  adc_attenuation_t attenuation_{ADC_0db};
+#endif
+};
+
+}  // namespace adc
+}  // namespace esphome