Add support for CSS embedded images/imagesheets

When displaying many emoji glyphs, it can be too many HTTP requests to load each glyph as a seperate image file.

This patch allows one to use CSS images loaded via class rules rather than specified inline, which is helpful if you want to do something like use a [CSS sheet generated by emojistatic](http://github.com/mroth/emojistatic) so that you only have a single HTTP request.

To use, specify `emoji.use_css_images = true;` and then as long as you have CSS rules loaded to handle the appropriate class names all is good!

I am using this on a (yet to be released) side project with these changes, figured it would make sense to merge them upstream for others to benefit and also to avoid maintaining a forked version.