initial interfaces for matching rtc API

Add interfaces for ```core.rtcdatachannel``` and ```core.rtcpeerconnection```. Eventually we will likely also want a ```core.mediastream``` api to implement ```getLocalStreams``` and friends.
Might be good to comment that this only reports channels opened by the remote peer, as per the spec (well, if the spec doesn't say so, then that's how it behaves in my experience).