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

app/models/admin_user.rb

@@ -0,0 +1,6 @@
+class AdminUser < ActiveRecord::Base
+  # Include default devise modules. Others available are:
+  # :confirmable, :lockable, :timeoutable and :omniauthable
+  devise :database_authenticatable, 
+         :recoverable, :rememberable, :trackable, :validatable
+end