Use PrettyPhoto jQuery plugin

[The PrettyPhoto jQuery plugin](http://www.no-margin-for-errors.com/projects/prettyphoto-jquery-lightbox-clone/) has several advantages over the current implementation of the gallery:
* Automatically adjusts to the viewport size (mobile users thank this), but has a toggle to show the image full size.
* Slideshow support
* Can display any type of content, not just images (good for scaling the gallery)
* No need to write 'overlay' markup
* More (see the project's documentation)

The license allows use "in all you projects even commercial ones as long as you leave the credits at the top [of the js file] intact." I've included all the theme files in my commit, in case you want to try them (just add theme:'theme_name' under line 16 of the template). If you don't use them, you can delete all theme folders (except default).