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.
I haven't quite understood what difference the priority option makes. No matter which setting i had chosen, the notification was always expanded (on Android 4.3). Does this option only affect certain versions of Android or have i overlooked something?