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.
> The resulting sent text would be invalid html, this is not a good way to do it haha.

I'm not suggesting you do this, but we didn't prevent before, so I don't think preventing it now is going to help anyone.

> (also it doesn't need to return itself because it is an endpoint)

I agree that there isn't a practical reason to return itself but to be consistent with `send()` it probably should. Eg. `send()` [returns itself](https://github.com/strongloop/express/blob/master/lib/response.js#L192).