Methods for modifying playlists

I've added three methods for modifying an existing playlist (see comments below). I also added the possibility to pass arbitrary params to `#user_playlist_tracks` like `limit` and `offset`.

I haven't had the time to include any specs yet as all of my currents ones are at a higher level in the application the code is currently being used in. The plan is to add some later tho, I just wanted to let you know that the code exists.