Issue #6: Login (and logout)

Completed this task.

Additional change: Changed Aobotracker::Application.config reference to Rails.application.config

We are putting some initializer code in config/initializers to hold configuration information.  In the code, the Config object is referenced via Aobotracker::Application.config, but in the application code, the shortcut to reference the Config object is just Rails.application.config.  Changed all references in application code (app/models, spec/factories, features/step_definitions, etc.) to use Rails.application.config.