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.
So, the second argument we've been passing is a little different to what you have here. DefinitelyTyped calls it `RTCMediaConstraints`:
https://github.com/borisyankov/DefinitelyTyped/blob/master/webrtc/RTCPeerConnection.d.ts#L68

See how it has two fields, `mandatory` and `optional`? Its `optional` field looks like what the spec calls `RTCOfferOptions`.