Modal fix

Makes off screen modals accessible by opening a non fixed window at the scroll offset position on screen. Fixes #87
Interesting. I believe this behavior is already handled in Foundation's CSS. For example, if you open the modal as we have it now on the demo site, this behavior is already present:

![responsive modal](http://g.recordit.co/PEP2PlSCnN.gif)

Was this not working properly with just the `scrollY` offset?