Adding check to remove user from channel when entering a new song from another.

* Adding loadCheck function to do the check and then call loadSong.
* Adding currentSong var to User to store the song the user is currently in
fixes bug in which entering a channel would erase the song data there
Fixes bug in which changing instrument would add another entry in FB instead of updating the existing one

* Split the updateSongData function into two functions, one for initializing data and one for updating
* Updating song data function can accept a lastInstrument parameter for updating the instrument.  If no param, just notes get updated.