Use TeXZilla when it is present (and MathJax absent).

This uses [TeXZilla](http://fred-wang.github.io/TeXZilla/) when it is present (for example if one includes http://fred-wang.github.io/TeXZilla/TeXZilla-min.js) to convert the labels to native MathML. This should be more standard, faster and could probably help with the styling of labels. However, this obviously requires a standard-compliant browser. I tested it with WebKit and Gecko with the files in examples/.