initial interfaces for matching rtc API
notes
update constructor per
https://github.com/borisyankov/DefinitelyTyped/blob/master/webrtc/RTCPeerConnection.d.ts#L44
use current offer constraint format, but remove from construtor
initial rtc implementation & demo
add tests
fix documentation ref to second constructor arg
Merge pull request #115 from freedomjs/rtcpeerconnection

initial interfaces for matching rtc API