build freedom-for-uproxy with our peerconnection-like core provider and sample app

This pull request:
* wraps `PeerConnection` as a Freedom module (`UproxyPeerConnection`)
* builds `freedom-for`uproxy`, with `UproxyPeerConnection` included as a core provider
* adds a sample app, `freedomchat` (identical to `chat`, except it uses Freedom)

I think the key thing to review is `UproxyPeerConnection`'s interface.

Note: I still need to add `freedom-for-chrome-for-uproxy` and `freedom-for-firefox-for-uproxy`. Otherwise, this request is more or less complete.