🏗 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/template/sensor/template_sensor.cpp

@@ -0,0 +1,28 @@
+#include "template_sensor.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace template_ {
+
+static const char *TAG = "template.sensor";
+
+TemplateSensor::TemplateSensor(const std::string &name, uint32_t update_interval)
+    : PollingSensorComponent(name, update_interval) {}
+void TemplateSensor::update() {
+  if (!this->f_.has_value())
+    return;
+
+  auto val = (*this->f_)();
+  if (val.has_value()) {
+    this->publish_state(*val);
+  }
+}
+float TemplateSensor::get_setup_priority() const { return setup_priority::HARDWARE; }
+void TemplateSensor::set_template(std::function<optional<float>()> &&f) { this->f_ = f; }
+void TemplateSensor::dump_config() {
+  LOG_SENSOR("", "Template Sensor", this);
+  LOG_UPDATE_INTERVAL(this);
+}
+
+}  // namespace template_
+}  // namespace esphome