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.
Resolving merge conflict.
Merge branch 'settings_ack' of https://github.com/MikeBishop/http2-spec into settings_ack