update to cohttp 0.12.0 api
regen oasis
Update to Cohttp.0.12.0 API

This is the API in [mirage/ocaml-cohttp#196]

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add opam file

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #19 from djs55/cohttp.0.12.0-api

Cohttp.0.12.0 api