Fix dependency on lwt.unix and release to OPAM

[aantron]

Markup.ml is not compatible with Lwt master, because it does not depend correctly on lwt.unix. cc @bluddy. See ocsigen/lwt#401 and ocsigen/lwt#374.

[aantron]

The diagnostics looked like this:

# + ocamlfind ocamlc -c -g -w +A-4-9-44-45-48 -warn-error +A-3 -bin-annot -safe-string -package lwt -package uutf -I src -I test -I doc -I test/performance -I test/pages -I test/js_of_ocaml -I test/dependency -o src/markup_lwt_unix.cmi src/markup_lwt_unix.mli
# File "src/markup_lwt_unix.mli", line 20, characters 14-41:
# Error: Unbound module Lwt_io
# Command exited with code 2.

I've also added a note to myself to mark existing Markup releases in OPAM as conflicting with Lwt >= 3.1.0, once that is nearing release.