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.
Right, it's somewhere inbetween -- Firefox and Chrome both implement `getStats()` but the output differs on both. FWIW, Chrome is moving towards the standard so I don't think we need to do anything as drastic as having different `peerconnection` providers for each browser.

I see three options:
1. have `getStats` return the raw stats object and leave processing to the client
2. have this raise an exception on Firefox, for now
3. make this work for Firefox, too

I don't even know if #1 is possible with Freedom? It's obviously not ideal although it would unblock me for now. If we do that, probably best to rename it somethinglike `getRawStats()` and consider omitting from the public interface for now.

Obviously, #3 is what we ultimately want. #2 might be a reasonable bridging step.