Various changes

- Added 404 controller (closes #3).
- Added 500 controller.
- Fixed MongoDB session error.
- Changed all register references to sign up instead.
- Fixed bug in controllersignupview.js.
- Redid some client routing stuff.
- Added models for category, forum, topic (topic is empty as a just
placeholder).
- Created basic category list on index.
- Started on category route.
- Completely abstracted all database functions.