🏗 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/hx711/hx711.cpp

@@ -0,0 +1,67 @@
+#include "hx711.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace hx711 {
+
+static const char *TAG = "hx711";
+
+void HX711Sensor::setup() {
+  ESP_LOGCONFIG(TAG, "Setting up HX711 '%s'...", this->name_.c_str());
+  this->sck_pin_->setup();
+  this->dout_pin_->setup();
+
+  // Read sensor once without publishing to set the gain
+  this->read_sensor_(nullptr);
+}
+
+void HX711Sensor::dump_config() {
+  LOG_SENSOR("", "HX711", this);
+  LOG_PIN("  DOUT Pin: ", this->dout_pin_);
+  LOG_PIN("  SCK Pin: ", this->sck_pin_);
+  LOG_UPDATE_INTERVAL(this);
+}
+float HX711Sensor::get_setup_priority() const { return setup_priority::DATA; }
+void HX711Sensor::update() {
+  uint32_t result;
+  if (this->read_sensor_(&result)) {
+    ESP_LOGD(TAG, "'%s': Got value %u", this->name_.c_str(), result);
+    this->publish_state(result);
+  }
+}
+bool HX711Sensor::read_sensor_(uint32_t *result) {
+  if (this->dout_pin_->digital_read()) {
+    ESP_LOGW(TAG, "HX711 is not ready for new measurements yet!");
+    this->status_set_warning();
+    return false;
+  }
+
+  this->status_clear_warning();
+  uint32_t data = 0;
+
+  for (uint8_t i = 0; i < 24; i++) {
+    this->sck_pin_->digital_write(true);
+    delayMicroseconds(1);
+    data |= uint32_t(this->dout_pin_->digital_read()) << (24 - i);
+    this->sck_pin_->digital_write(false);
+    delayMicroseconds(1);
+  }
+
+  // Cycle clock pin for gain setting
+  for (uint8_t i = 0; i < this->gain_; i++) {
+    this->sck_pin_->digital_write(true);
+    this->sck_pin_->digital_write(false);
+  }
+
+  data ^= 0x800000;
+  if (result != nullptr)
+    *result = data;
+  return true;
+}
+HX711Sensor::HX711Sensor(const std::string &name, GPIOPin *dout, GPIOPin *sck, uint32_t update_interval)
+    : PollingSensorComponent(name, update_interval), dout_pin_(dout), sck_pin_(sck) {}
+
+void HX711Sensor::set_gain(HX711Gain gain) { this->gain_ = gain; }
+
+}  // namespace hx711
+}  // namespace esphome