🏗 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/rdm6300/rdm6300.cpp

@@ -0,0 +1,69 @@
+#include "rdm6300.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace rdm6300 {
+
+static const char *TAG = "rdm6300";
+
+static const uint8_t RDM6300_START_BYTE = 0x02;
+static const uint8_t RDM6300_END_BYTE = 0x03;
+static const int8_t RDM6300_STATE_WAITING_FOR_START = -1;
+
+void rdm6300::RDM6300Component::loop() {
+  while (this->available() > 0) {
+    uint8_t data;
+    if (!this->read_byte(&data)) {
+      ESP_LOGW(TAG, "Reading data from RDM6300 failed!");
+      this->status_set_warning();
+      return;
+    }
+
+    if (this->read_state_ == RDM6300_STATE_WAITING_FOR_START) {
+      if (data == RDM6300_START_BYTE) {
+        this->read_state_ = 0;
+      } else {
+        // Not start byte, probably not synced up correctly.
+      }
+    } else if (this->read_state_ < 12) {
+      uint8_t value = (data > '9') ? data - '7' : data - '0';
+      if (this->read_state_ % 2 == 0) {
+        this->buffer_[this->read_state_ / 2] = value << 4;
+      } else {
+        this->buffer_[this->read_state_ / 2] += value;
+      }
+      this->read_state_++;
+    } else if (data != RDM6300_END_BYTE) {
+      ESP_LOGW(TAG, "Invalid end byte from RDM6300!");
+      this->read_state_ = RDM6300_STATE_WAITING_FOR_START;
+    } else {
+      uint8_t checksum = 0;
+      for (uint8_t i = 0; i < 5; i++)
+        checksum ^= this->buffer_[i];
+      this->read_state_ = RDM6300_STATE_WAITING_FOR_START;
+      if (checksum != this->buffer_[5]) {
+        ESP_LOGW(TAG, "Checksum from RDM6300 doesn't match! (0x%02X!=0x%02X)", checksum, this->buffer_[5]);
+      } else {
+        // Valid data
+        this->status_clear_warning();
+        const uint32_t result = (uint32_t(this->buffer_[1]) << 24) | (uint32_t(this->buffer_[2]) << 16) |
+                                (uint32_t(this->buffer_[3]) << 8) | this->buffer_[4];
+        bool report = result != last_id_;
+        for (auto *card : this->cards_) {
+          if (card->process(result)) {
+            report = false;
+          }
+        }
+        for (auto *trig : this->triggers_)
+          trig->process(result);
+
+        if (report) {
+          ESP_LOGD(TAG, "Found new tag with ID %u", result);
+        }
+      }
+    }
+  }
+}
+
+}  // namespace rdm6300
+}  // namespace esphome