Minor update of Ed's draft

Dear TF,

the getusermedia draft has received a very minor update: the different 
components (audio, video) that you want to capture are now opt-in, 
rather than opt-out (the discussion on the list indicated that this 
behavior was preferred). The examples are modified accordingly.

(Another thing that should be updated is the title, but that is not done 
in this version.)

<http://dev.w3.org/2011/webrtc/editor/getusermedia.html> (dated version 
at <http://dev.w3.org/2011/webrtc/editor/getusermedia-20111222.html>)

Stefan