Fixes #198 with an Add to New Playlist button.

Added an Add to New Playlist button. It calls the SavePlaylist()
Javascript function that fetches the song_id from the jplaylist
on the client side, then makes an Ajax call.

I had to modify the song item_type so that the item_id would be
all added accordingly. Adding a new item_type didn't work for
unknown reason, the new item_type case never being picked up.