big fix and tweaks
major design changes. most of media related functionality moved to a new singleton class
fixed song selection issue after rework
added new buttons, reworked MainActivity

- main activity now sports a static resetUI() function which guarantees
  that no buttons will be in invalid states, and that no loss of activity
  focus will effect the text views and otherwise

- removed the onMediaEvent function in main activity, now just call
  resetUI() for all intents and purposes

- added next and previous buttons and assets, added new methods to support these
  buttons

- renamed callback functions, retuned the main layout a bit

- private button helper methods no longer require arguments, they figure
  things out themselves by using the singleton as necessary
added next/previous functionality

- created new media controls in MediaState singleton which allow for
  advancing/falling back a track on the current playlist.

- "SongCompletionListener"'s job moved to the nextTrack() method, will
  likely remove class and turn it into a anonymous inner class