Added carouselTopBottomPadding option

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:

![photoswipe2](https://f.cloud.github.com/assets/1431130/1394569/81d7ae56-3c2b-11e3-9ca4-92099880c583.png)


And here's the one with carouselTopBottomPadding specified :
![photoswipe](https://f.cloud.github.com/assets/1431130/1394391/d433fd4c-3c28-11e3-975c-2edd77db0af0.png)

Here's an example usage:

    var myPhotoSwipe = $("#Gallery a").photoSwipe({ carouselTopBottomPadding : 60 });