Rework ORTA/ORTV. Fixes #66, #64.

ORTA/ORTV can now be set to a number smaller than the number of MSTs,
leading to a=sendonly settings.
Also allowed VAD to work in CreateAnswer. (#64)
I agree that this is what we should be aiming for, but I'm not in love with the text.

Perhaps: 
If N is set to a value less than M, the offer MUST mark as sendonly
the the most recent M-N of the m= sections added since the most recent
call to setLocalDescription.  This allows the offerer to indicate that
it does not want to receive audio on some or all of its newly created
streams.  If fewer than M-N m= sections have been added since
setLocalDescription, then all such sections are marked as sendonly;
any m= sections added prior to setLocalDescription remain unchanged.