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.
well, it'll set it to `text/html`, so it should get set to `test/plain` in here and then `res.send`.

i.e. the body of this should probably be
```js
this.type('txt');
this.status(statusCode);
this.send(http.STATUS_CODES[statusCode]);
```