Add simple POST body example for async

As asked by @loldrup
Update _oasis for example
Flush after writing full response

Fixes #179
impl. of transfer encoding was duplicated
Merge pull request #184 from rgrinberg/remove_duplicate_code

impl. of transfer encoding was duplicated
Rename Conduit_resolver to Resolver to match Conduit 0.6+ API
Merge pull request #185 from avsm/master

Rename Conduit_resolver to Resolver to match Conduit 0.6+ API
Merge pull request #183 from rgrinberg/fix-179

Flush after writing full response
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
sync oasis
Propagate the Net.ctx type equivalance for Cohttp_lwt.Make_client as well
Improve ocamldoc for Cohttp.Accept
Improve ocamldoc for Cohttp.Accept_types
Rename `Cohttp.Auth.t` to `Cohttp.Auth.resp` to make it match the
existing `Cohttp.Auth.req` type.  Also expose an `Other` variant
to make it more extensible for unknown authentication types.
Travis: temporarily deactivate building Irmin (known-broken)
Fix test case for 889e1914ff13e04aa680aab24bc5d049fc04b6f6 change
Cohttp_lwt_unix: expose the ctx type equivalence so that custom resolvers can be built
Add test case for custom resolver