Add support for additional SDP attributes needed for WebRTC interoperability.

I'm working on a [XMPP/Jingle WebRTC](https://github.com/legastero/stanza.io) library, and originally made an [SDP/JSON converter](https://github.com/legastero/sdp) for that, but I like yours a lot more :)

These attributes are ones that I have so far found to be required for reliable WebRTC SDP exchanges between FireFox and Chrome.