add centering to reveal

There used to be the ability to add fixed in reveal to vertically center just through CSS (see: http://foundation.zurb.com/forum/posts/783-zurb-reveal-doesnt-take-into-account-scrolling).

When you add the fixed css to the reveal-modal class, it still has to deal with 

    $(window).scrollTop() + el.data('css-top') + 'px';

Which, when dealing with fixed elements, will give you a number that'll shoot the modal well past the viewport, which needs a different scale. 

It accounts for the possibility that the 

   Math.floor ($(window).height() - el.height())/2

could be lower than el.data('css-top'), because you wouldn't want the possibility of a negative top with the modal regardless of view size.

Tested with both pop & fade, and set the params usually.