🏗 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/bh1750/bh1750.cpp

@@ -0,0 +1,81 @@
+#include "bh1750.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace bh1750 {
+
+static const char *TAG = "bh1750.sensor";
+
+static const uint8_t BH1750_COMMAND_POWER_ON = 0b00000001;
+
+BH1750Sensor::BH1750Sensor(const std::string &name, uint32_t update_interval)
+    : PollingSensorComponent(name, update_interval) {}
+
+void BH1750Sensor::setup() {
+  ESP_LOGCONFIG(TAG, "Setting up BH1750 '%s'...", this->name_.c_str());
+  if (!this->write_bytes(BH1750_COMMAND_POWER_ON, nullptr, 0)) {
+    this->mark_failed();
+    return;
+  }
+}
+void BH1750Sensor::dump_config() {
+  LOG_SENSOR("", "BH1750", this);
+  LOG_I2C_DEVICE(this);
+  if (this->is_failed()) {
+    ESP_LOGE(TAG, "Communication with BH1750 failed!");
+  }
+
+  const char *resolution_s;
+  switch (this->resolution_) {
+    case BH1750_RESOLUTION_0P5_LX:
+      resolution_s = "0.5";
+      break;
+    case BH1750_RESOLUTION_1P0_LX:
+      resolution_s = "1";
+      break;
+    case BH1750_RESOLUTION_4P0_LX:
+      resolution_s = "4";
+      break;
+    default:
+      resolution_s = "Unknown";
+      break;
+  }
+  ESP_LOGCONFIG(TAG, "  Resolution: %s", resolution_s);
+  LOG_UPDATE_INTERVAL(this);
+}
+
+void BH1750Sensor::update() {
+  if (!this->write_bytes(this->resolution_, nullptr, 0))
+    return;
+
+  uint32_t wait = 0;
+  // use max conversion times
+  switch (this->resolution_) {
+    case BH1750_RESOLUTION_0P5_LX:
+    case BH1750_RESOLUTION_1P0_LX:
+      wait = 180;
+      break;
+    case BH1750_RESOLUTION_4P0_LX:
+      wait = 24;
+      break;
+  }
+
+  this->set_timeout("illuminance", wait, [this]() { this->read_data_(); });
+}
+float BH1750Sensor::get_setup_priority() const { return setup_priority::DATA; }
+void BH1750Sensor::read_data_() {
+  uint16_t raw_value;
+  if (!this->parent_->raw_receive_16(this->address_, &raw_value, 1)) {
+    this->status_set_warning();
+    return;
+  }
+
+  float lx = float(raw_value) / 1.2f;
+  ESP_LOGD(TAG, "'%s': Got illuminance=%.1flx", this->get_name().c_str(), lx);
+  this->publish_state(lx);
+  this->status_clear_warning();
+}
+void BH1750Sensor::set_resolution(BH1750Resolution resolution) { this->resolution_ = resolution; }
+
+}  // namespace bh1750
+}  // namespace esphome