Issue #98: Back should exit playback not move to next video

* Check the requestCode and the resultCode.  Don't play next video.
* If normal playback is selected, clear the existing video queue.
* Display toast message when video queue has been cleared ahead of playback
* Display toast message when video has been exited and there are still videos
  in the queue.
Issue #98: Add optional support for external playback continuous play.

Adds optional support for continuous play from an external player.  Play
back will continue regardless if back button is pressed and will continue
until the queue is empty.
Issue #98: Fix issue with video playback queue and external players

When using an external player other than MX Player there was an issue in
which the video play back queue was not being used.  Needed to use the
method startActivityForResult instead of just startActivity.