Add an assertion that checks how much a value has changed.

This would be used to check if, when something is invoked, something else is changed by x.

For example:

``` javascript
var someArray = [1, 2, 3];

expect(function(){
  someArray.pop();
}).to.change(someArray, 'length').by(-1);
```

I'd be glad to take a stab at this.
