Make audio unit channel class more amenable to subclassing

Michael, I'd be interested in hearing your thoughts about merging this upstream. 

We found that adding these two protected instance variables to the header allowed us to make more useful subclasses (e.g. a streaming file player with precise timing).

Did you perhaps intend the ```preInitializeBlock:``` initializer parameter to be a workaround to providing protected access to the audio unit? Seems like that would be somewhat hackier than just exposing it to subclasses.