Updated charter proposal

The charter task force met a bit over a week ago and had a productive
discussion that led to a handful of changes being made to the proposal.

You can take a look at the resulting charter here:
http://juberti.github.io/webrtc-charter/proposed-charter.txt

Diffs from my previous proposed charter:
https://github.com/juberti/webrtc-charter/compare/59a3bce69b5a74b052b8519c314d7191d99ece19...juberti:alt