Add support for CSS embedded images

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!