Close sio

When creating a new Server with a port

```js
var PORT = 3030;
var sio = require('socket.io')(PORT);
```

socket.io is creating a new http server https://github.com/Automattic/socket.io/blob/master/lib/index.js#L209 when only sending a PORT. This will allow to close this server 

Currently 

```js
var PORT = 3030;
var sio = require('socket.io')(PORT);

sio.engine.close();

var sio = require('socket.io')(PORT);

  Error: listen EADDRINUSE
```
This could technically be used on someone's machine. We need to use ephemeral ports in tests.