added: MediaItemTag for ManagedMediaSources.
added: silent track for FailedMediaSource.
added: keyframe fast forward at initial playback buffer.
added: error notification on silently skipped streams.
-Extracted version numbers in gradle dependencies.
-Updated ExoPlayer to 2.7.1.
-Updated RxJava to 2.1.10, RxAndroid to 2.0.2 and RxBinding to 2.1.1.
-Removed deprecated implementation of media buttons.