Default 404 page

This release adds another error page; the 404 File Not Found. Along with the recently added error codes, this should make it a lot easier to debug proxy issues / log misbehaving clients or the like.

Instead of simply getting a RST (which often times results in 502), proxies and end clients now get the proper 404 error code for any unhandled URL / method.

This new behavior can be tested by running the EchoServer example and requesting any HTTP page.