Remove ability to call once() without a parameter

Code such as 

```

// Now change the background color.
$('p').once().css('background-color', 'black');
// Change it again.
$('p').once().css('background-color', 'yellow');
```

Doesn't actually help anything. ID are required to control what's going on.

Related #15 
Could we have a test to expect an error when there is no `id` parameter?