Remove error code of INVALID_STREAM

It is no longer used after d5a8faeaa605bdff7b96287d72913b4b742104cf
Drop SETTINGS persistence as discussed at the interim
Specify how to handle PING frames of lengths that aren't 8.

Specifically calls them out as connection errors of type
PROTOCOL_ERROR.
Updating references to retrying sample.
Oxford comma.
Fixing upgrade transition semantics.  Closes #52.
Small editorial fixes.
Clarify the interaction between SETTINGS and flow control.
Merge pull request #134 from jpinner/end_flow_control

Clarify the interaction between SETTINGS and flow control.
Merge pull request #125 from jasnell/settingspersist

Drop SETTINGS persistence
Removing settings flags, marking the field as reserved.  Closes #8.
Merge pull request #117 from shigeki/shigeki_20130612_3

Remove INVALID_STREAM error code
Merge branch 'PING' of git://github.com/willchan/http2-spec into willchan-PING
Editorial changes to PING frame description.
Clarifying that the PRIORITY frame does not define any type-specific flags.
Merge pull request #135 from molnarg/priority-frame-flags

Clarifying that the PRIORITY frame does not define any type-specific flags
Reverse the polarity of the CONTINUES flag.
Merge pull request #136 from jpinner/flag_polarity

Reverse the polarity of the CONTINUES flag.  Apply consistent names to flags.  Closes #129.