Cache css attribute background-image

Allows caching of images inserted by css stylesheets as `background-image`. 

Example:
```
<div id="test_div" style="background-image: url(http://url/image.png);">TEST</div>

<script>
ImgCache.cacheBackground($('#test_div'));
ImgCache.useCachedBackground($('#test_div'));
</script>
```