Add the figaro gem
Move omniauthable links to another place
Add the capybara gem
Remove comments
Add support for logging in with Facebook

* add and configure omniauth-facebook with devise
  * add new columns to the User model, :provider and :uid
  * update config/application.yml.example with stub for the facebook
    credentials
* add the 'Log in with Facebook' button on the /users/sign_in page
* add a spec