Add ejaydj codes

Codes added from my gem: https://github.com/ejaypcanaria/ejaydj for review purpose only.
Ah, I think I got your point here. It looks like `all_playlists` is going to get each playlists' track. I made `tracks` on `Playlist` lazy loaded so it wont get it immediately. If there's a scenario where in there are still tracks on a playlist (a `track` is popped out from a playlist every time `next_track` is called) the user won't get the reloaded tracks from Spotify.

`playlist.reload!` is going to force that even if there are still available tracks.