Support for "Thumbs Up" playlist

The get_thumbs_up_tacks() call returns a list of tracks that have
thumbs up (like) being set. This endpoint is also used be mobile
clients to show the automatic playlist "Thumbs up"/"Mag ich".
Changed get_thumbs_up_tracks() to get_thumbs_up_songs()

This is for consistency with get_all_songs(). In addition, the term
song better represents the entirety of the data (audio + meta).
Added a test for get_thumbs_up_songs() which currently fails
Fixed the test for get_thumbs_up_songs()

The method get_thumbs_up_songs() only applies to All Access tracks
being rated.
Added a comment to get_thumbs_up_songs()
Merge pull request #264 from hechtus/feature/ThumbsUpPlaylist

add support for "Thumbs Up" playlist