proxy rewrite

If you enable -DLWS_WITH_HTTP_PROXY=1 at cmake, the test server has a
new URI path http://localhost:7681/proxytest If you visit here, a client
connection to http://example.com:80 is spawned, and the results piped on
to your original connection.

Also with LWS_WITH_HTTP_PROXY enabled at cmake, lws wants to link to an
additional library, "libhubbub".  This allows lws to do html rewriting on the
fly, adjusting proxied urls in a lightweight and fast way.