custom status

i think this is enough for custom status.

```js
var koa = require('koa');
var app = koa();

app.statuses['700'] = 'Custom Status';

app.use(function* () {
  this.status = 700;
});

app.listen(3000);
```

```
curl http://127.0.0.1:3000/ -i
HTTP/1.1 700 Custom Status
X-Powered-By: koa
Content-Type: text/plain; charset=utf-8
Content-Length: 13
Date: Wed, 01 Oct 2014 12:27:11 GMT
Connection: keep-alive

Custom Status
```
labels just need tweaking since they mention res.statusMessage still