Add ocaml-io-page

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
io-page depends on mirage-types; rename to .spec

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #41 from djs55/io-page

Add ocaml-io-page and ocaml-mirage-types