Uniquify spotify playlist names

Implements https://github.com/mopidy/mopidy/issues/114#issuecomment-15396025


 * Appends the playlist owner to those playlists owned by other users i.e. followed playlists
  * Left playlists owned by the current Spotify user alone - presumably they shouldn't appear any different to LocalBackend playlists
 * Appends an incrementing number to playlists with the same name
  * First occurrence of a name is unchanged
  * Looking again I've realised this isn't the place to do this when dealing with multiple backends, perhaps I should remove this bit?