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, I see I messed up the constructor in the doc I sent you.

We've been args to the constructor which are quite different from what's in the spec. Here's how DefinitelyTyped defines it:
```typescript
declare var RTCPeerConnection: {
  prototype: RTCPeerConnection;
  new (configuration: RTCConfiguration,
       constraints?: RTCMediaConstraints): RTCPeerConnection;
};
```

adapter.js expects this signature, too, as do most samples I've seen.

I'm not sure what the exact story is here; I can play around with this and see what happens in FF and Chrome and get back to you.