Added carouselTopBottomPadding option

[uhaciogullari]

This commit introduces an option to add top and bottom padding to image carousel.

Caption and toolbar will never be hidden in my case and I needed to display the image between toolbar and caption.

Here's the default behavior:

And here's the one with carouselTopBottomPadding specified :

Here's an example usage:

var myPhotoSwipe = $("#Gallery a").photoSwipe({ carouselTopBottomPadding : 60 });