Add DTMF handling for '*' (repeat menu) and '#' (return to menu/exit)

Adds support for the following DTMF buttons:

'*' - repeats the last menu.
'#' - exits from the change folders prompt. If already in the main menu, plays bye and hangs up.
Looks like there's already a hangup method defined on the finite state machine that performs a hangup. You could call self.hangup() to do this. Also, if you want the action to be performed regardless of the success of the promise returned by play, you can use .finally(function() {self.hangup();});