auto embedding of pictures in css

it needs more testing but I thought I'd create that pull request anyway to see what others think about this.

5.6.1 has a lot of great performance improvements but I thought about other ways to improve the performance and came up with an idea to reduce the number of http request.

I'm doing this by including pictures in the css. Simply set EMBED_CSS_IMAGES to true in site.php and all your pictures are inline in the css.