mathjax support

Screen shot
![screen shot 2014-04-01 at 6 05 18 pm](https://cloud.githubusercontent.com/assets/1690993/2586543/e2a5ca92-ba02-11e3-8055-0dd80a16e6b7.png)

This PR depends on the my PR #62. I didn't split the two PRs because if I do so, the two PRs will be conflicting each other. If you like this PR and don't like #63, let me know, I can prepare a different PR.

For those who are interested in it, remember to add

```
<script type="text/x-mathjax-config">
  MathJax.Hub.Config({
    tex2jax: {
      inlineMath: [ ['$','$'], ["\\(","\\)"] ],
      processEscapes: true
    }
  });
</script>
<script type="text/javascript"
  src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
</script>
```

in your `head.html` template file.