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

db/seeds.rb

@@ -5,3 +5,4 @@
 #
 #   cities = City.create([{ name: 'Chicago' }, { name: 'Copenhagen' }])
 #   Mayor.create(name: 'Emanuel', city: cities.first)
+AdminUser.create!(email: 'admin@example.com', password: 'password', password_confirmation: 'password')