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.
:+1:  to static constructors.

Two suggestions:
* Both the static constructor and constructor accept a string called `ref` so it's not clear what translation is being done. I guess this `ref` is the Freedom channel ID?
* `make_____` isn't totally accurate since this doesn't actually *make* the channel. If this were Java-land I'd name it `fromId` or similar. Thoughts?