Add session management:
  * Create sessions controller & helper to handle user session
    management
  * Add user model, with andrewid and is_admin attributes
  * Add sessions resource routes and login & logout named routes
  * Add filter methods require_authentication &
      require_administrator to application controller
  * Add require_authentication filter to vote, nominations, and
      candidates controller

Update application layout to display flash[:error] in addition
to flash[:notice].