Style with tabs to find documents more easily

This is the implementation to fix #4 .

Using Tikz David has created a good looking page layout that can contain categories and includes tabs to make it easier to identify documents.

Some features are still missing:

- [ ] Category macros (e.g. \doctheorie)
- [ ] Version number (major and minor) support (e.g. \docversion{1}{3} -> v1.3)
- [ ] HiSPARC logo on the first page