🏗 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/hmc5883l/sensor.py

@@ -0,0 +1,71 @@
+# coding=utf-8
+import esphome.codegen as cg
+import esphome.config_validation as cv
+from esphome.components import i2c, sensor
+from esphome.const import CONF_ADDRESS, CONF_ID, CONF_RANGE, CONF_UPDATE_INTERVAL, \
+    ICON_MAGNET, UNIT_MICROTESLA, UNIT_DEGREES, ICON_SCREEN_ROTATION
+
+DEPENDENCIES = ['i2c']
+
+hmc5883l_ns = cg.esphome_ns.namespace('hmc5883l')
+
+CONF_FIELD_STRENGTH_X = 'field_strength_x'
+CONF_FIELD_STRENGTH_Y = 'field_strength_y'
+CONF_FIELD_STRENGTH_Z = 'field_strength_z'
+CONF_HEADING = 'heading'
+
+HMC5883LComponent = hmc5883l_ns.class_('HMC5883LComponent', cg.PollingComponent, i2c.I2CDevice)
+
+HMC5883LRange = hmc5883l_ns.enum('HMC5883LRange')
+HMC5883L_RANGES = {
+    88: HMC5883LRange.HMC5883L_RANGE_88_UT,
+    130: HMC5883LRange.HMC5883L_RANGE_130_UT,
+    190: HMC5883LRange.HMC5883L_RANGE_190_UT,
+    250: HMC5883LRange.HMC5883L_RANGE_250_UT,
+    400: HMC5883LRange.HMC5883L_RANGE_400_UT,
+    470: HMC5883LRange.HMC5883L_RANGE_470_UT,
+    560: HMC5883LRange.HMC5883L_RANGE_560_UT,
+    810: HMC5883LRange.HMC5883L_RANGE_810_UT,
+}
+
+
+def validate_range(value):
+    value = cv.string(value)
+    if value.endswith(u'µT') or value.endswith('uT'):
+        value = value[:-2]
+    return cv.one_of(*HMC5883L_RANGES, int=True)(value)
+
+
+field_strength_schema = sensor.sensor_schema(UNIT_MICROTESLA, ICON_MAGNET, 1)
+heading_schema = sensor.sensor_schema(UNIT_DEGREES, ICON_SCREEN_ROTATION, 1)
+
+CONFIG_SCHEMA = cv.Schema({
+    cv.GenerateID(): cv.declare_variable_id(HMC5883LComponent),
+    cv.Optional(CONF_ADDRESS): cv.i2c_address,
+    cv.Optional(CONF_FIELD_STRENGTH_X): cv.nameable(field_strength_schema),
+    cv.Optional(CONF_FIELD_STRENGTH_Y): cv.nameable(field_strength_schema),
+    cv.Optional(CONF_FIELD_STRENGTH_Z): cv.nameable(field_strength_schema),
+    cv.Optional(CONF_HEADING): cv.nameable(heading_schema),
+    cv.Optional(CONF_UPDATE_INTERVAL, default='60s'): cv.update_interval,
+    cv.Optional(CONF_RANGE, default='130uT'): validate_range,
+}).extend(cv.COMPONENT_SCHEMA).extend(i2c.i2c_device_schema(0x1E))
+
+
+def to_code(config):
+    var = cg.new_Pvariable(config[CONF_ID], config[CONF_UPDATE_INTERVAL])
+    yield cg.register_component(var, config)
+    yield i2c.register_i2c_device(var, config)
+
+    cg.add(var.set_range(HMC5883L_RANGES[config[CONF_RANGE]]))
+    if CONF_FIELD_STRENGTH_X in config:
+        sens = yield sensor.new_sensor(config[CONF_FIELD_STRENGTH_X])
+        cg.add(var.set_x_sensor(sens))
+    if CONF_FIELD_STRENGTH_Y in config:
+        sens = yield sensor.new_sensor(config[CONF_FIELD_STRENGTH_Y])
+        cg.add(var.set_y_sensor(sens))
+    if CONF_FIELD_STRENGTH_Z in config:
+        sens = yield sensor.new_sensor(config[CONF_FIELD_STRENGTH_Z])
+        cg.add(var.set_z_sensor(sens))
+    if CONF_HEADING in config:
+        sens = yield sensor.new_sensor(config[CONF_HEADING])
+        cg.add(var.set_heading_sensor(sens))