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)
Fixed typo that made Travis sad
Fix errata noticed by Eric
Merge pull request #84 from rtcweb-wg/sendonly

Rework OTRA/OTRV. Fixes #66, #64.