Add code -> phrase mapping

[LeTraceurSnork]

Please add some int code -> string phrase mapper so we could use smth like

$some_code = \Teapot\StatusCode\Http::HTTP_NOT_FOUND;

echo \Teapot\StatusCode\Http::getReasonPhrase($some_code); // Not Found
echo \Teapot\StatusCode\Http::getReasonPhrase(404); // Not Found