Improved cache buster handling for font URLs

This is an improvement/addition to 727ac80a677af21856de55f575b54387b7b53503.

Font URLs do often contain query strings and/or hashes (e.g. "example.eot?#iefix" or "example.svg#my-font"). This patch adds support for cache busters in combination with such font URLs.

Besides that, I changed the default value of font_url's :cache_buster argument to true, just like with image_url. I think that's more consistent.