Adding extensions back
Clarifies duplicate allowance in "sets".
Improve explanations about memory limitations
Clarifies differences between literal representations.
Misc editorial improvements
Removing EXTENSION frame
Updating based on interim discussion
Changing IANA policy for frame types
Merge pull request #493 from http2/extend

Adding extensions back.  Closes #95.
make the XML valid
Restricting to AEAD modes only.