🏗 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/ble_rssi/ble_rssi_sensor.h

@@ -0,0 +1,42 @@
+#pragma once
+
+#include "esphome/core/component.h"
+#include "esphome/components/esp32_ble_tracker/esp32_ble_tracker.h"
+#include "esphome/components/sensor/sensor.h"
+
+#ifdef ARDUINO_ARCH_ESP32
+
+namespace esphome {
+namespace ble_rssi {
+
+class BLERSSISensor : public sensor::Sensor, public esp32_ble_tracker::ESPBTDeviceListener, public Component {
+ public:
+  BLERSSISensor(const std::string &name, esp32_ble_tracker::ESP32BLETracker *parent, uint64_t address)
+      : sensor::Sensor(name), esp32_ble_tracker::ESPBTDeviceListener(parent), address_(address) {}
+
+  void on_scan_end() override {
+    if (!this->found_)
+      this->publish_state(NAN);
+    this->found_ = false;
+  }
+  bool parse_device(const esp32_ble_tracker::ESPBTDevice &device) override {
+    if (device.address_uint64() == this->address_) {
+      this->publish_state(device.get_rssi());
+      this->found_ = true;
+      return true;
+    }
+    return false;
+  }
+  void setup() override { this->setup_ble(); }
+  void dump_config() override;
+  float get_setup_priority() const override { return setup_priority::DATA; }
+
+ protected:
+  bool found_{false};
+  uint64_t address_;
+};
+
+}  // namespace ble_rssi
+}  // namespace esphome
+
+#endif