Added Action Button support and misc fixes

SDK:
* Now supports action buttons.
* Fixed issue where notifications from the same app where getting replaced instead of adding to the notification area.
* Main Android Activity is now brought to the foreground instead of a new one being created when opening a notification.
* Misc code clean up.

Example project:
* Removed android.permission.GET_ACCOUNTS permission from example as our code is using the correct GoogleCloudMessaging class which does not require this.
* Fixed AlertDialog to still work when a new instance of the MainActivity is created and also on Android 2.3 devices when a notification is opened.