Fix to overlay problem on some stack orders

The current overlay backdrop doesn't work right if your stack order is using 3d transformations (i.e. iScroll). So, Instead of using the standard overlay, create 4 boxes around the elements and use that for the overlay. It's supported as an optional parameter and it defaults to doing it the original way.

Also, Added a new "showBackdrop" option which works around the problem where the popover appears below the backdrop.