Use ENV variables for configuration.

* Use ENV variables with magiconf configuration
* Use dotenv (https://github.com/bkeepers/dotenv) for setting ENV variables locally
* Remove the application.example.yml in favor of a .env.example
* Updates the README to explain that dotenv is now being used

Pivotal Tracker ID: [64152486](https://www.pivotaltracker.com/s/projects/984852/stories/64152486)

This should make deploying the application to a hosting PaaS, like Heroku, easier.