Add res.sendStatus method

A helpful method for apps that send many simple status code responses, e.g., REST APIs. The body of the response is set using node's http.STATUS_CODES to avoid problems with clients that choke on empty body responses.
> If you're going to add a new API, I recommend doing it correctly from the start.

Sure. How about... if `req.is('html')` we send back valid HTML with some basic formatting?