Added django_extensions so a graphical map of the models can be generated.

The file is myapp_models.pdf.  You can regenerate it anytime with:  

```python manage.py graph_models -a -o myapp_models.pdf```

I also switched to sqlite3 because getting the MySQL extension working is annoying in many platforms (especially OS X), and it still had Python 3 problems the last time I checked.  You can roll this back but count me as at least one person for whom it was an obstacle.  

Also, you need to pip install django_extensions before you can use the graph_models command.