Playlists are not createable via "new" anymore. This induced a bug where
new playlists would be created when the V8 object of a NodePlaylist was
created.

Moved the cache for playlists from the PlaylistContainer to the Playlist
class itself.

The ready callback is now fired in the spotify login callback.
Deletion of playlists added
Refactored callbacks to take a variable number arguments in C++. Playlist adding event propagated to javascript