a=imageattr negotiation specification

This change adds a=imageattr generation, parsing, and guidelines for usage.
I think that the only reasonable thing to do is what you always do with SDP: share what you are willing to receive.  And then the same rules apply in both directions equally.

Sure, the answerer has the advantage of knowing what the offerer is willing to receive, but I'm going to recommend that they don't alter their recv attributes at all.