Adds tag instead

Adds a bit of denormalization (main_mandate) and adds select_related()
to query_set to reduce lazy loading in loops

- Cleans core app by removing useless files
- Organizes utils in a utils directory
- Creates a digg-like pagination class
http://blog.localkinegrinds.com/2007/09/06/digg-style-pagination-in-
django
- Adds constance to installed app to manage settings in django admin
console
- Adds debug_toolbar and django_extensions in dev mode