Add 'Categories:' click-able links on waterfall page

This patch adds 'Categories:' click-able links on 'Waterfall' page.
These links are convenient way to filter waterfall by categories without manually put  '/waterfall?category=...' in  the address bar.

Screenshot:
http://pix.academ.org/img/2014/07/10/40be2295c6e0de74ef17b8e138f5421c.png
(This is a valid Python, though) I believe these three lines could be replaced by

```python
ctx['categories'] = sorted(categories)
```