Port WebRtc.PeerConnection to freedom-0.6's core.rtcpeerconnection.

This is a very rough draft with regard to code style, but it does
include a version of simple-freedom-chat that uses the new
WebRtc.PeerConnection.  This is working in Chrome, but not yet in
Firefox.
Cool. Seems like we'll have a little work to do updating the typings. That's a separate issue. We already have an issue filed for this.