Adds the Unicorn Config file

I noticed that Unicorn was being used in the Gemfile but there was no Unicorn config file --  https://devcenter.heroku.com/articles/rails-unicorn

* Adds the Unicorn config file
* Adds Unicorn config file to startup using the `Procfile` and foreman
* Updates the `README` to reflect changes on how to start the server.