Introduce Promise's in gUM draft

As already announced [1] we think there is consensus to introduce 
Promise’s for navigator.mediaDevices.getUserMedia, applyConstraints and 
enumerateDevices while there is still no clear consensus on how backward 
compatibility should be specified.

To help us move forward we’ve asked the Editor’s to start updating as 
follows:

* introduce Promise’s for navigator.mediaDevices.getUserMedia, 
applyConstraints and enumerateDevices
* document navigator.getUserMedia with callbacks, but add a note saying 
there is still not a clear consensus on this part

Stefan for the chairs

[1] 
http://lists.w3.org/Archives/Public/public-media-capture/2014Oct/0162.html