Improved diff rendering

This improves rendering of a single article history diff. Features are:

* togglably collapsed rendering of unchanged areas
* no more ugly underlines in \<ins\>-tags
* togglable mathjax rendering, use "mathjax": True in a template's context to enable mathjax
* diff is now on article markdown/LaTeX source, not on the outputted HTML.

![image](https://f.cloud.github.com/assets/578029/601658/8d7758e4-cc85-11e2-9e0d-4061ce6a7241.png)
A diff with some changes and a collapsed area.

![image](https://f.cloud.github.com/assets/578029/601662/b1bcff4c-cc85-11e2-9a5e-42c488fdb0a2.png)
The same diff with the middle area expanded.

This fixes #43 and #53.