Added Active Admin

Please set the environment variable ADMIN_EMAILS when you deploy. The value of the variable should be a comma separated list of email ids you want to add as admin users.
For now just add yourself

ex: 
heroku config:add ADMIN_EMAILS=me@example.com

Also, when you are testing in your local environment, set the environment variable before you start your server. Once you login, you should see a navigation menu called "Admin".