Add persistent notification with play/pause/stop buttons

Creates additional buttons for play, pause, and stop in the notification. 

Options for: 
 - Keep notification when paused.
 - Auto expand the notification.

Addresses #405 except for lock screen.
Although it doesn't make any functional difference, i'd recommend using the constants [NotificationCompat.PRIORITY_DEFAULT](http://developer.android.com/reference/android/support/v4/app/NotificationCompat.html#PRIORITY_DEFAULT) and PRIORITY_MAX here, so that it is clear what those numbers mean.