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.
Rather than window.setTimeout, just setTimeout should be used. This way we don't assume the existence of the window object.