Adding "data-description" attribute support.

With this change it is possible to specify an optional "data-description" attribute. If not empty a new div on the right of the image will appear, containing the specified description. My needed was to create something like the facebook photogallery, mantaining the style of lightbox and its semplicity. I tried to respect the coding style of the rest of the project. I am sorry if I made something wrong, this is the first time I contribute to an open source project. :)