Support events fired at the root object

[remy]

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

Something like:

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?

[adelriosantiago]

How could this be implemented? Any idea someone?