Add tag cloud.

Hi,
I've created a simple tag cloud for the fresh theme (see the screenshot below). The font size/weight scaling may need some tuning, I'd be happy to hear your opinion! 
Cheers,
david
![tag-cloud](https://f.cloud.github.com/assets/514775/923341/42ee16e4-ff45-11e2-9678-1d545e407829.png)