Fixing bug in which switching songs while playback is on caused playback to continue and couldn't be turned off

* Creates new function for turning off all notes that are set to be on
* Calling allNotesOff from loadSong
* Pulls control of playback from the Controls listener function and makes a new function specfiically for controlling playback.
* playbackControl is called from loadSong
* Removed extra code from playNotes - was doing an extra loop of assigning instruments to channels - unnecessary since it can be done inside the loops for playing notes
Merge pull request #54 from artfoundry/fix-playback-control-bug

Fixing bug in which switching songs while playback is on caused playback...