Added devise, cancan and omniauth

Added the following gems:

  * devise
  * devise-encryptable
  * cancan
  * omniauth
  * omniauth-facebook
  * omniauth-twitter

The vikingo-rails-config Thor task will perform the following modifications to
the new project, after integrating the database-related gems:

  * Initialize devise
  * Add a devise pepper file
  * Make changes on app/controllers/application_controller.rb
  * Configure base generators
  * Add a devise pepper to the heroku configuration