🏗 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/codegen.py

@@ -0,0 +1,26 @@
+# Base file for all codegen-related imports
+# All integrations should have a line in the import section like this
+#
+# >>> import esphome.codegen as cg
+#
+# Integrations should specifically *NOT* import directly from the
+# other helper modules (cpp_generator etc) directly if they don't
+# want to break suddenly due to a rename (this file will get backports for features).
+
+# pylint: disable=unused-import
+from esphome.cpp_generator import (  # noqa
+    Expression, RawExpression, TemplateArguments,
+    StructInitializer, ArrayInitializer, safe_exp, Statement,
+    progmem_array, statement, variable, Pvariable, new_Pvariable,
+    add, add_global, add_library, add_build_flag, add_define,
+    get_variable, process_lambda, is_template, templatable, MockObj,
+    MockObjClass)
+from esphome.cpp_helpers import (  # noqa
+    gpio_pin_expression, register_component, build_registry_entry,
+    build_registry_list)
+from esphome.cpp_types import (  # noqa
+    global_ns, void, nullptr, float_, bool_, std_ns, std_string,
+    std_vector, uint8, uint16, uint32, int32, const_char_ptr, NAN,
+    esphome_ns, App, Nameable, Trigger, Action, Component, ComponentPtr,
+    PollingComponent, Application, optional, arduino_json_ns, JsonObject,
+    JsonObjectRef, JsonObjectConstRef, Controller, GPIOPin)