🏗 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).

tests/test2.yaml

@@ -2,11 +2,6 @@ esphome:
   name: $devicename
   platform: ESP32
   board: nodemcu-32s
-  # Use this for testing while developing:
-  # Note the travis check for esphome PRs will fail until the
-  # esphome-core PR has been merged.
-  # esphome_core_version:
-  #   local: ~/path/to/esphome-core
   build_path: build/test2
 
 substitutions:
@@ -142,7 +137,7 @@ esp32_touch:
   setup_mode: True
 
 binary_sensor:
-  - platform: esp32_ble_tracker
+  - platform: ble_presence
     mac_address: AC:37:43:77:5F:4C
     name: "ESP32 BLE Tracker Google Home Mini"
   - platform: esp32_touch
@@ -201,6 +196,8 @@ text_sensor:
           variables:
             my_variable: |-
               return id(version_sensor).state;
+            my_variable_str: |-
+              return "Hello World";
   - platform: template
     name: "Template Text Sensor"
     lambda: |-