hologram styleguide

This adds [hologram](trulia.github.io/hologram/) to adhocracy. Hologram is a tool to automatically create a breakdown (styleguide) of available UI widgets from CSS comments. This is meant as a better alternative to the existing `templates/debug/components.html`.

I already transferred some of the code examples from `components.html` to CSS comments. I guess it is not important to migrate everything in one step.