fullscreen mode : fix for when image is larger than window and added feature to specify fullsize image

This pull requests consists in 2 parts:

- fix for when image is larger than window (added demo to illustrate the problem)
- added feature to specify fullsize image (using data-fullimg-href tag, to specify an extra fullsize image to use when fullscreen is specified)

The second feature is useful for galleries that have a fullsize and resize images, by default the resize is shown and if user requests fullscreen the fullsize is loaded.

I have tested this in latest Firefox and Chromium in Linux.

I have modified source/jquery.fancybox.js but not the packed file. How is the packed file generated exactly?