Update WINDOW_UPDATE from HTTP/2.0 draft spec.

Key change is addition of new flag for ending flow control for the
entire session or a specific stream.
Merge branch 'WINDOW_UPDATE' of https://github.com/willchan/SPDY-Specification into gh-pages

Conflicts:
	draft-mbelshe-spdy-00.html
	draft-mbelshe-spdy-00.txt