Add font mimetype to avoid warnings on exercise pages.

Added a mimetype for OTF fonts, to avoid warnings like:

    Resource interpreted as Font but transferred with MIME type application/octet-stream:     
    "http://localhost:1234/static/js/khan-exercises/utils/MathJax/1.1a/fonts/HTML-CSS/TeX/otf/MathJax_Math-Italic.otf".