Implement RTCMedia extensions

Implement the new RTCMedia extensions to the RTCPeerConnection interface. https://w3c.github.io/webrtc-pc/archives/20151222/webrtc.html#rtp-media-api
This appears to be the way WebRTC will be going in general. Better to get this implemented sooner rather than later.  Fortunately, these appear to be mostly if not fully implemented by the native WebRTC API.
