Add CSS for MathML fallback

This is related with [Bug 981409](https://bugzilla.mozilla.org/show_bug.cgi?id=981409).

## Short Description

Some browsers don't support MathML and because of this the sections in MDN that cover MathML will be bad render. As example, visit https://developer.mozilla.org/en-US/docs/Web/MathML/Examples/MathML_Pythagorean_Theorem with Firefox (that support MathML) and with Chrome (that **don't** support MathML).

@fred-wang [wrote a small CSS](https://github.com/fred-wang/mathml.css) to "fix" the lack of MathML support for basic math expressions and ask me to add it into kuma.

## Examples

View of MDN sample with this PR in Firefox:

![screenshot from 2014-03-26 14 52 24](https://f.cloud.github.com/assets/1506457/2528576/fb9bde3e-b510-11e3-8558-7681e01434a7.png)

View of MDN sample with this PR in Chrome:

![screenshot from 2014-03-26 14 52 13](https://f.cloud.github.com/assets/1506457/2528581/0248aafa-b511-11e3-923e-c1fbad71a1db.png)