support selectable categories for tutorials

[erikerlandson]

The concept is something like how category tags operate in octopress:
http://erikerlandson.github.io/blog/archives/

where you can select a category and get the blogs that were tagged with that. In this case, you could select all tutorials tagged with 'kafka' or 's2i', etc

[elmiko]

i think it would be cool if we could have the tagging extend over the tutorials and the projects as well