Create device model and setup devise, activeadmin and rspec!

- with attributes: model, imei_number, unique_id and access_token
 - access_token is automatically generated on create

config/routes.rb

@@ -1,4 +1,6 @@
 Rails.application.routes.draw do
+  devise_for :admin_users, ActiveAdmin::Devise.config
+  ActiveAdmin.routes(self)
   # The priority is based upon order of creation: first created -> highest priority.
   # See how all your routes lay out with "rake routes".