Explicitly flush the debug output when the `COHTTP_DEBUG` env variable is set
Add Async.Client head/post/patch/delete methods.

Modify the test case to terminate, and try a few different sites.
[lwt] Do not send chunked encoding headers with GET/DELETE requests that have no body.
[lwt] Client.head no longer returns a response body, just the metadata.
add test case for 6c90286fbfc013035f7a0c20696b74e2c8a16fd2

closes #29
oasis sync