[API] Implement register/unregister global desktop keyboard shortcut.

* Introduce `GlobalShortcutListener`;
* New api object: Shortcut({'key': '...', 'active': ..., 'failed': ...});
* New api: App.registerGlobalHotkey, App.unregisterGlobalHotkey.

Usage demo can be found here: [tests/manual_tests/global_hotkey](https://github.com/zhchbin/node-webkit/tree/hotkey/tests/manual_tests/global_hotkey)

Fix #200 and thanks @hokein for the MAC part.

@rogerwang PTAL.