Add PostgreSQL command line client

This changeset installs the PostgreSQL command line client on the application server so that it can be used to interact with the database.

```
vagrant@app:~$ envdir /etc/nyc-trees.d/env/ /opt/app/manage.py dbshell
Password for user nyc_trees:
psql (9.3.5)
SSL connection (cipher: DHE-RSA-AES256-SHA, bits: 256)
Type "help" for help.

nyc_trees=#
```

Attempts to resolve #72.