DTMF - the "Object Oriented" approach

Takeaways from Lyon were that:

- Executing DTMF needs reference to an audio track (to know where to 
send the data) and to a PeerConnection (to know that we've successfully 
negotiated use of the DTMF codec).
- The WG preferred an "object oriented" model: creating a DTMF handler 
object, rather than the "fortran" approach of having all functions 
directly on the PeerConnection.

Suggested edits, delta from the October 19 version of the spec:

- In section 8.4, rename AudioMediaStreamTrack to 
DTMFSendingMediaStreamTrack.
Add the following text:

A RTCDTMFSendingMediaStreamTrack is created by calling the 
createDTMFSender() method on a PeerConnection. This constructs an object 
that decorates a MediaStreamTrack with the functions required to send DTMF.


- In section 4.3.2, add the function

RTCDTMFSendingMediaStreamTrack createDTMFSender(MediaStreamTrack track);

- In section 4.3.2.2, add the paragraph

createDTMFSender

The createDTMFSender() creates an RTCDTMFSendingMediaStreamTrack that 
references the given MediaStreamTrack. The MediaStreamTrack MUST be an 
element of a MediaStream that's currently in the PC's localStreams 
attribute; if not, throw an Illegal Argument Exception. [NOTE - get 
correct name for exception before inserting]