Install pg for development

Changed the development and test databases from Sqlite to Postgres.
This file should probably be renamed to `config/database.example.yml`, and `config/database.yml` should be added to `.gitignore`. That way individual developers can name their dev & test databases differently, or use different creds.

Also, the `production` section is unnecessary, since it is ignored by Heroku.

One other note: You can set up defaults in yaml! It's a nice trick for shortening up these files and making them more readable.

``` yaml
development: &default
  adapter: postgresql
  encoding: utf8
  database: philly-open-health_development
  pool: 5
  username: 
  password:

test:
  <<: *default
  database: philly-open-health_test
```