playback: Add empty playback provider
playback: Add threadsafe BufferTimestamp wrapper
playback: Add MUSIC_DELIVERY event listener
playback: Add END_OF_TRACK event listener
playback: Add resume() method
playback: Add stop() method
playback: Add empty play() method
playback: Make play() load and play Spotify track
playback: Make play() setup appsrc
playback: Pass need_data/enough_data callbacks to appsrc
playback: Wire up seek_data callback from audio
playback: Update timestamp and seek in Spotify on seek_data callback from audio
playback: Ignore first seek to zero after play, ref mopidy/mopidy#300
playback: Music delivery is rejected if we don't want more audio data
playback: Music of unknown audio formats are rejected
playback: Deliver audio buffers to mopidy.audio