a=imageattr negotiation specification

This change adds a=imageattr generation, parsing, and guidelines for usage.
Surely we can say that implementations can include what they are able to send.  Obviously, what they actually send will be determined by what the remote peer can receive, but the SDP doesn't need any more complications.

p.s. Harald, when you reply to emails like this, I have a really hard time separating response from quoted part.  I find that it's easier to just use the github interface for these.