Change addStream to addTrack; add RTCRtpSender/RTCRtpReceiver

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.

Details:
* addStream/remoteStream/getSenders/getReceivers and associated APIs have now been moved to a new document section (#5). This allows section #4 to focus on the basics of the PC API, and #5 to extend it with media functionality and #6 to extend it with data channel functionality.
* addStream has become addTrack(MediaStreamTrack track, MediaStream... streams), as discussed in DC. addTrack returns a RTCRtpSender.
* removeStream has become removeTrack; removeTrack takes a RTCRtpSender.
* onaddstream has become onaddtrack; onremovestream has been removed (now, .onended is fired).
* getLocalStreams/getRemoteStreams -> getSenders/getReceivers.
In the discussion about changeTrack we had a while back I think we concluded that one parameter here should be trackId. Perhaps we should not add that now, but in conjunction with introducing changeTrack later.