Added mathjax support

I added support for mathjax by changing the rendering from redcarpet to kramdown, and adding
a script tag to the template.
I updated the bezier page to use mathjax instead of the ad hoc math text.
The change of renderer doesn't seem to break anything, except some tabular syntax used for
the ad-hoc math, which didn't seem to render correctly anyway (for example in the math page).
These math sections need to be replaced by the mathjax syntax.
Can you move that in the footer at the end of the page? – This would prevent halting DOM rendering for browsers that don’t play well with async loading (which would need the HTML5 `async` attribute, btw).