Created playback package inside service package
Moved background tasks of PlaybackService into PlaybackServiceTaskManager
Added first version of PlaybackServiceMediaPlayer
Added tests for PlaybackServiceTaskManager
Added basic tests for PlaybackServiceMediaPlayer
Made PlaybackService use the PSMP and PSTM
Adapted remaining classes to new playback service model
Updated video player UI
Use AspectRatioVideoView instead of standard video view
Applied bugfix from pull-request #313
Avoid seeking to position 0 (#208).
Improved error handling in MediaPlayerActivity
Bugfix: Video was paused when started
Resolved problems with ending playback.

- OnPreparedListener removed: not necessary anymore, because end of preparing playback is handled manually
Fixed issue with switching between audio and video player
PLAY and PLAY_PAUSE keycode was not handled correctly in prepared state
Bugfixes

- Skipping an episode did not work
- pauseOnHeadsetDisconnect preference was not updated correctly
Merge branch 'playbackservice_rewrite' into develop
Fixed race condition when sleep timer expires
Updated version numbers in build.gradle