add the timtumb php class to work with images

Class as found on: http://timthumb.googlecode.com/svn/trunk/timthumb.php
show thumbnails on preview page to allow easy browsing

Only shows the first 16 thumbnails (a number you can easily change).
Generating the thumbnails for the first time takes a bit of time,
but they are cached for a month after being generated.

We'll show thumbnails only for the most recent images.

You need to install php5-gd for this to work:
apt-get install php5-gd