Pagination For Playlist Tracks

Attempting to work out a pattern that can be applied elsewhere as needed for https://github.com/guilhermesad/rspotify/issues/8

Doesn't really have a way to get at a specific page yet, but allows to page forward and backward through a list of tracks in a playlist.  I mixed in enumerable to preserve current array functionality of playlist.tracks, but this is kind of limiting and confusing.  Maybe could keep tracks method as a normal array and make a new method that is more clear that is for pagination like ```playlist.tracks_page``` or something?  Then that could potentially take offset and limit values if you want to start in the middle.