Category filtering for Planet

I wrote a patch to Planet ( a blog aggregator) to implements filtering based on category. This was my first Python hacking to add a new feature to an existing code base. Yah!

It will only filter based on the "primary category" as that is the only category information available in the RDF feed.

We will need this functionality when deploying the Planet on the main Guru Labs web page.

I just submitted the patch to the main developers. Hopefully they'll accept it and in the future we can just use a stock Planet install.