Add simple POST body example for async

As asked by @loldrup
Update _oasis for example
Merge branch 'async_post_example' of git://github.com/rgrinberg/ocaml-cohttp into rgrinberg-async_post_example
travis: clone mirage-www directly
Merge pull request #186 from avsm/rgrinberg-async_post_example

Update #182 with oasis sync