Add 422 Unprocessable Entity from WebDAV (RFC 4918) for ocaml-github
Merge pull request #22 from dsheets/422

422 Unprocessable Entity
sync CHANGES
add Lwt/Mirage build rules back
Add user agent string and User-Agent header helper function
Merge avsm:master and fix CHANGES typo
Add Header.replace function; drop UA version
Merge branch 'user-agent' of git://github.com/dsheets/ocaml-cohttp into dsheets-user-agent

Conflicts:
	CHANGES
mark 0.9.7
sync build, including a `-short-paths` hack for trunk ocaml
lift out Response.r into a type independent of IO
lift Request out of IO
remove the parameterised monads for now, while simplifying the Client interface
revert to a working Async without Cohttp.Client
move request encoding logic back into Request, outside the monad
remove much of the Client functor
cleanup Request encoding (still more work needed)
easier to read lwt debugging
fixes to get Lwt compiling again with new Cohttp internals
flesh out the async client again