fixes
playlist: ensure trailing slash on subscribed uris
playlist: move createPlaylist from Spotify object into Playlist object as a static method

Wraps the Playlist object class inside Spotify object instances,
allowing the use of the Playlist constructor and static methods on the
Playlist object (Spotify instance argument done automatically by _wrap
function).
Updates the examples to indicate the new way of using the Playlist API
(i.e. using the wrapped objects).