Remove djcelery dependency

This PR removes the django-celery dependency, replacing the `djcelery.setup_loader()` call with the recommended setup derived from [Using Celery with Django](http://celery.readthedocs.org/en/latest/django/first-steps-with-django.html).

The most important line is the one that allows celery to auto-discover tasks based on the installed apps. In run_tests.py:

```python
app.autodiscover_tasks(lambda: settings.INSTALLED_APPS)
```

Additionally, I've made it possible to run the tests locally in a virtual environment without having to install postgres. They can be run this way with the command:

    export DB='sqlite'; python setup.py test