Allows containment to be a function.

With containment being a function users can do more complex usage of the control. I added an example to the index.html where 2 handlers cannot cross each other.

![screen shot 2014-05-08 at 11 59 26 am](https://cloud.githubusercontent.com/assets/9697/2919373/86ee187c-d6da-11e3-8e12-1d91c86454ca.png)