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.
I guess we should reuse the [old implementation](https://github.com/strongloop/express/blob/master/lib/response.js#L107).

```js
res.sendStatus = function sendStatus(status) {
  if (!this.get('Content-Type')) {
    this.type('txt');
  }

  this.statusCode = status;
  this.send(http.STATUS_CODES[status] || String(status));

  return this;
}
```

Lets you specify the type:

```js
res.type('html').sendStatus(500);
```