a=imageattr negotiation specification

This change adds a=imageattr generation, parsing, and guidelines for usage.
imageattr is actually two attributes on one line: send and recv.
the imageattr RFC says things that make it seem like what's in an offer is a proposal, and what's in the answer is the responder picking from what's in the proposal.
For the offerer sending a recv and the answerer sending a send, that's fairly simple - answerer can decide what to send, and then announce that.
For the offerer sending a send and the answerer sending a recv, I find it harder to say what's right. The RFC says that it is legal to replace the section completely, but says "the implementor of this feature should however be aware that this may cause extra offer/answer exchanges".
But the text becomes MUCH simpler if the answerer simply ignores the incoming "send", so I think I'll do that