Added in music and sound effects for their demo.
Added in a music button to access the demo.
Added in volumes to the AudioManager.  (Will add in setter functions later.)
Added in new functionality to Scene where play [song name] [number] will play the song and stop it after [number] second(s).
Added in isPlaying boolean into Sound to make Stop and Pause work properly.
Added in StopAfter function into Song to make it stop after the given time.
Added in Caller variable into Timer to make calling functions on the caller easier.