Implement global shortcut API, fixes #439

@zcbenz, please take a look! 
And thanks for @orzFly on windows platform implementation.
Source code put under `chromium_src` should be put as exactly at where it was in chrome, so the files should be put under `chromium_src/chrome/browser/extensions` instead of `chromium_src/chrome/browser/ui/shortcut`, and we shouldn't rename their namespace from `extensions` to `atom::api`.

This is important when we want to sync the source code with chrome in future.