Adding to change log.
Merge branch 'layering' of github.com:http2/http2-spec into layering
Tweaking header block text.
Moving stuff to HttpExtra section.  Clarifying, rewording and reformatting server push section.  Removing HEAD push, for now.
Formatting tweaks.
Merge branch 'layering' of github.com:http2/http2-spec into layering
some minor editorial clean-up
Clarifying examples.  Editorial fixups.
Editorial cleanup, courtesy @jpinner.