web server stop functions now take a name
add behavior for underlying web servers

Add the webmachine_server behavior module, which underlying web servers
must implement. Refactor webmachine_request module to rely on this new
behavior, with the largest change being due to avoiding the assumption that
the underlying web server exposes its socket (cowboy doesn't). Move common
recv and send code from webmachine_request to webmachine_server so that it
can be shared by web servers that can expose their sockets.

Update cowboy to 0.8.6.

Modify some tests to use ephemeral ports.