custom satus

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
```
Here seems still need to config custom status code before calling, and that is not cool. i think module `statuses` is no necessary to be visible to the users by additional api `app.statuses`.
Should we just allow:
```js
this.status = 700;
this.statusMessage = 'custom status';
// or
this.setStatus(700, 'custom status');
```
or something like these?

Otherwise, for RFC status is one thing:
```js
this.status = 200;
this.statusMessage = 'GOOD';
```
for custom status is another thing:
```js
app.statuses['700'] = 'custom status';
app.use(function *(){
  this.status = 700;
})
```
is that weird?