* Add constants for sensor device_class
* Add device_class attribute to sensor component
* Add device_class attribute to sensor class
* Add device_class to mhz19 temperature sensor
* Add device_class to sensor in api component
* Add test for device_class of sensor
* Rename DEVICE_CLASS_NONE to DEVICE_CLASS_EMPTY for consistency
* Make optional attributes of sensor component truly optional
## 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).
Paul Monigatti