Create Async port

[johnelse]

Async should be an optional dependency.

[cfcs]

If you're going to support Async / Lwt, please see conduit:
https://github.com/mirage/ocaml-conduit/

[johnelse]

Indeed, I'd like to switch to conduit when I have some spare time. I'm also considering dropping the Unix variant, as everyone I've spoken to seems to be using Lwt :)