Wire up the database

This changeset connects the Django application (running on `app`) to the
PostgreSQL database (running on `services`). The Django application is
setup to pull database settings in from the environment and `envdir` is
used to populate the environment.

Example usage:

```bash
$ vagrant ssh app
vagrant@app:~$ envdir /etc/nyc-trees.d/env /opt/app/manage.py runserver
```

Attempts to resolve the following issues:

- https://github.com/azavea/nyc-trees/issues/28
- https://github.com/azavea/nyc-trees/issues/31
- https://github.com/azavea/nyc-trees/issues/35
- https://github.com/azavea/nyc-trees/issues/36