Lazy load images

[vletroye]

For galleries with 10th of photos, it's nice to lazy load them. I.e.: load them only when the user scrolls to their locations. Doing so, "image_big" is loaded "earlier" (it's indeed often only loaded when all the photos are loaded in the "linear_pane").

I did integrate this plugin http://jquery.eisbehr.de/lazy in a local copy of Photoshow and it gives quite good results, making the user experience smoother.

[vletroye]

I did fork the master branch and have create a Pull Request with, among others, that improvements: #355