add getStats() to core.peerconnection

This adds a function named `getStats` to `core.peerconnection` which exposes a subset of `RTCPeerConnection.getStats`'s output, namely the local and remote peer-to-peer connection endpoints.

I haven't added tests yet -- am keen to get your thoughts first. I think in future we'll want to add other fields exposed by `getStats` such as bytes sent, jitter, etc. etc.

BTW, I guess we could just add this to the provider (as discussed during the hackathon) but I figure this will be useful to others, too, so long as we note that it's still somewhat experimental.
Well, I just figured it's better than polling.