Playlist support

Adds a bunch of new functionality for working with Playlists. Some TODOs remain. 

Includes methods to create, modify and delete* playlists, add tracks and remove tracks from playlists.

Playlists are now returned as a Playlist object**, similar to Track, Artist and Album objects.

Includes a simple example that creates a new playlist and adds some songs to it in `example/createPlaylist.js`

Includes #20

Fixes #18, #19

'* Spotify never actually deletes playlists, instead it just removes them from the user's rootlist (and if it's their playlist sets the deletedByOwner attribute to true).

** Playlist object is a bit weird, especially in the way it inherits from the SelectedListContent schema. Please check that it is OK before committing.