Background images added

I've added the ability to lazyload images that are added via the background-image selector. I've been using lazyload for a while and with every project I've needed this, so I decided just to do it myself. A couple of points:

- Animations with background images do not work. This was not something I personally needed, but I'm guessing it can be done, even if just via CSS animations.

- Images are loaded via the "data-original" attribute. This attribute is read and converted into an inline style call to background-image.

- I also created a demo page called background_image.html.