ACK flag and use for Header Table size updates

Adds the ACK flag to SETTINGS with a section on settings
synchronization.  Moves the SETTINGS_HEADER_TABLE_SIZE setting into the
core spec, and replaces it with a brief recap of how synchronization
applies to compressor context.  Also renames the PONG flag for
consistency.
Marking for implementation.
Updating ALPN version.
Removing implementation draft notice, will restore after IETF Vancouver.
[#250] remove substitution from HPACK
Merge pull request #262 from jpinner/header_updates

[#250] remove substitution from HPACK
Renaming frame size error; using it consistently.  Fixes #246.
Merge branch 'master' of github.com:http2/http2-spec
Proposal for #257.
Closes #263.
Adding constraints to push.  Fixes #193.
Restricting frame length to 2\^14.  Closes #260.
Adding restrictions on colon header usage.  Closes #221.
420 Enhance Your Calm Error Code
Merge pull request #265 from jpinner/420_enhance_your_clam

420 Enhance Your Calm Error Code
s/:host/:authority/g.  Fixing this for CONNECT.  Closes #268.
Merge branch 'master' of github.com:http2/http2-spec
fix P6 reference
fix anchor
chmod -x