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.
Should there be some text about the effect putting streams here will have (that the onaddtrack even on the remote side will fire with these streams)?