getUserMedia default values

Hi

I think it would be nicer to specify what kind of media your application 
wants rather than what it doesn't want. E.g., with the current spec the 
following applies:

navigator.getUserMedia({audio:false}, cb); // means video
navigator.getUserMedia({audio:true}, cb); // means audio and video

With the current "opt-out" approach your application will also ask for 
permission to use all future media types (which are hard to opt-out from 
before they exist).

/Adam