Support events fired at the root object

There's no mapping for the root object, so maybe this is a special key? 

Something like:

``` js
var o = new Bind({
  cats: ['sam', 'nap', 'prince']
}, {
  ':root': callback
});

Akin to the [:root](http://www.w3.org/TR/css3-selectors/#root-pseudo) CSS selector.

Thoughts?
```
