Switch to "listener" scheme instead of override for on...Message

[robotconscience]

Up for discussion, but I think we should switch to the scheme we used in the past to add listeners to functions instead of just overriding onBooleanMessage, etc:

// now
sb.onBooleanMessage = myBooleanFunction;

// future
sb.onBooleanMessage( myBooleanFunction );

// OR
sb.addListener( sb.onBooleanMessage, myBooleanFunction );