Separated a special NodeWrapped class for objects with callbacks as not all (e.g. tracks) need them
Reenabled the bridge between playlists and NodePlaylists, but still fragile
NodePlaylists nulls the nodeObject in Playlist if deleted
Callbacks are now called in the fashion of function(err, object) instead of mutating this
Made all the functions in node-spotify.cc static to not pollute the global namespace
Some documentation on the NodeWrapped base classes