Continue implementation of SpotifySession singleton class

After a bit of research, there are three possible solutions to solving my problem of safely notifying the receiver of a session of success or failure:

* using delegate and datasource protocols to notify the receiver of success or failure as well as to actually provide the requested session
* using NSNotificationCenter (from what I've read, this would be very bad form for this situation)
* using a completion handler; this would involve quite a complicated series of logical statements.

I've therefore decided to go with implementing protocols, as this seems to be the most elegant solution.