Issue #259 - Embed images in CSS so asset fingerprinting works.

Moves images into base64-encoded CSS to fix problems when using the Rails asset pipeline and asset fingerprinting. Changing an image is almost as easy as it was: instead of changing the image in the filesystem, you simply base64 encode it and paste it into the CSS. Use an encoder such as this one: http://webcodertools.com/imagetobase64converter