async: reimplementation using openflow.async backend

This library is really out-of-date. It depends on a very old version of
ocaml-openflow, which is no longer supported. So this commit
reimplements the library using the openflow.async library, which is
current and supported.

Closes #1
Closes #4
async: add core as a dependency

core is a dependency of async, so this is somewhat redundant, but
explicit's sometimes better than not.
async: add travis files
async: use opam-street for travis
async: don't test on ocaml < 4.01.0

openflow does not support previous versions.
Merge pull request #5 from frenetic-lang/async

reimplementation using openflow.async backend