More optimizations

 *  Ranch update to 0.8.0, custom supervisor, much faster to process incoming connections.
 *  Cowboy update to 0.8.3, configurable stream chunk length (fixes a @todo I left earlier).
 *  Avoid using set_resp_body/2, start sending files directly with the response, sometimes saving one packet on the whole file. This should make a visible difference only with small to medium files.

Please tell me if anything's wrong.