New directory layout

- Most things are now camelCase.
- Fix category id being displayed in article lists.
- Fix new articles not having an author.
Move two files to their correct location
Fix categories class
Move some pages over to mvc

I'm still experimenting with mvc, code should be improved later on. It's
also difficult sometimes to glue old code with new code, without
rewriting everything at once.
Change directory structure again
Add link to category of each article