Finishing off "rollback"

We've discussed this at length, but now it needs to get written into the
WebRTC spec. JSEP describes the rationale for rollback and how it ought to
work in http://tools.ietf.org/html/draft-ietf-rtcweb-jsep-06#section-4.1.4.2,
briefly stated as:
- Rollback discards any proposed changes and returns the session to the
stable state; localDescription and remoteDescription are restored to their
previous values.
- Rollback can only be used to cancel proposed changes; you can't go back
to a previous stable state.
- Rollback is performed by supplying a SessionDescription of type
"rollback", with empty contents, to the setLD/setRD that was most recently
used (if the offer to be rolled back was supplied to setRD, the rollback
description needs to be supplied to setRD as well).

I'd also like to suggest that we limit rollback to only be valid in the
have-local-offer/have-remote-offer states, given the limited utility of
rollback in pranswer states, as well as the fact that we are trying to
discourage use of pranswer.