Fixed Shadow Filter on Mobile Devices

Fixed a Bug on Mobile Devices, which can be seen here:
https://github.com/lokesh/lightbox2/issues/336
![bug](https://cloud.githubusercontent.com/assets/9317809/4705756/f0f0ea58-587e-11e4-94f0-079d28495b6f.png)


The width for the black filter(lightboxOverlay) was set to <code>window.width</code>. The problem is that the mobile Devices scale the website smaller, so you have to use <code>document.width</code>, which is always the entire screen. 

Aswell there is a bug with position absolute on mobile devices which can be seen here:
http://stackoverflow.com/questions/16518334/using-position-absolute-in-mobile-responsive-design

You have to have a parent element with <code>position:relative</code> on a mobile device. Otherwise <code>position:absolute</code> will break.