Add RTCRtpSender, RTCRtpReceiver, and friends

Initial version of doohickeys. @alvestrand @stefhak @pthatcherg PTAL and see if you think this is going in the right direction. If so I will deal with the few formatting and link nits remaining.
If addTrack is called with more than one stream (such as pc.addTrack(track, stream1, stream2), which stream is in the event that gets fired?  Or does more than one event get fired?