Add support for starred playlists

I ran into a problem where I had a user and wanted to grab tracks from his playlists, but I was getting 404 Resource Not Found for the "Starred" playlist. Turns out that this playlist has a different path than normal playlists. 

What I did was instead of constructing the playlist endpoint url for each call, use the href field already provided by Spotify. Also added specs and cleaned up a bit.

I did not change the methods add_tracks!, change_details! and replace_tracks! because I had no easy way to test it. I'm pretty sure it would be fine to use the href but I wanted to discuss and get an ok before changing it without specs.

What do you think?