Initial library retrieval working. Playlist support temp removed.

-Using Mobileclient API for library retrieval - many of Webclient's
 methods have been deprecated in the latest versions of gmusicapi.
-Minor change to GBaseSource.init_songs() to accomodate keys returned by
 Mobileclient.get_all_songs()
-Commented-out code that would load playlist data for the time being -
 Playlist data from the Mobileclient is MUCH different than the Webclient's.
Playlist retrieval working, sort-of.

Track metadata isn't in Mobileclient.get_all_user_playlist_contents().
Need to make the GPlaylist.init_songs() get song metadata from the
RhythmDB that gets populated in the first place.
Playlist track metadata lookup implemented.

GPlaylist.init_songs() now looks up songs already entered into our local
RhythmDB to fill in missing playlist track info.
Merge pull request #43 from p0lar-bear/api_update_fix

Api update fix