Choosing overlay's parent element.

I added a new option called `overlayParent` that allows the user to choose the element that will contain the overlay `div`.

It might be helpful in situations where the user does not have control under the html. 

In my case, I needed it while working on a project written in  asp.net Webforms.

As this new option does not change the current default behaviour, I thought it might be worth sending you a pull request.

I wrote a [gist](https://gist.github.com/Asakeron/5522805) with a working example.