Allow to use chunked encoding with Hunchentoot

This allows to use chunked encoding with Hunchentoot if a function is
provided as the body value of the reply, and if the header Transfer-Encoding
is set to chunked. The function is then called with the output stream as
argument.

Signed-off-by: Julien Danjou <julien@danjou.info>
Merge pull request #56 from jd/jd/chunked

Allow to use chunked encoding with Hunchentoot