🏗 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/helpers.py

@@ -1,5 +1,6 @@
 from __future__ import print_function
 
+import codecs
 import logging
 import os
 
@@ -143,3 +144,41 @@ def get_bool_env(var, default=False):
 
 def is_hassio():
     return get_bool_env('ESPHOME_IS_HASSIO')
+
+
+def copy_file_if_changed(src, dst):
+    src_text = read_file(src)
+    if os.path.isfile(dst):
+        dst_text = read_file(dst)
+    else:
+        dst_text = None
+    if src_text == dst_text:
+        return
+    write_file(dst, src_text)
+
+
+def read_file(path):
+    try:
+        with codecs.open(path, 'r', encoding='utf-8') as f_handle:
+            return f_handle.read()
+    except OSError:
+        from esphome.core import EsphomeError
+        raise EsphomeError(u"Could not read file at {}".format(path))
+
+
+def write_file(path, text):
+    try:
+        mkdir_p(os.path.dirname(path))
+        with codecs.open(path, 'w+', encoding='utf-8') as f_handle:
+            f_handle.write(text)
+    except OSError:
+        from esphome.core import EsphomeError
+        raise EsphomeError(u"Could not write file at {}".format(path))
+
+
+def write_file_if_changed(text, dst):
+    src_content = None
+    if os.path.isfile(dst):
+        src_content = read_file(dst)
+    if src_content != text:
+        write_file(dst, text)