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.
@bemasc can weigh in, but in my conversation with him this morning he said neither of the srtp keys are needed to be passed since they are now the default behavior in the browsers we're targeting.