improve resolver_lwt_unix ocamldoc
Better error message from Conduit TLS match failure
Update the index.html docs
Sync changes
Update README
Add a `CONDUIT_DEBUG` environment variable to the Unix backends for live debugging
Expose the representation of `Conduit_lwt_unix.flow` in the external signature.

This lets library users obtain the original `Lwt_unix.file_descr` when using
Conduit libraries like Cohttp.  See mirage/ocaml-cohttp#188 for one such
request.