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/admin/admin_user.rb

@@ -0,0 +1,28 @@
+ActiveAdmin.register AdminUser do
+  permit_params :email, :password, :password_confirmation
+
+  index do
+    selectable_column
+    id_column
+    column :email
+    column :current_sign_in_at
+    column :sign_in_count
+    column :created_at
+    actions
+  end
+
+  filter :email
+  filter :current_sign_in_at
+  filter :sign_in_count
+  filter :created_at
+
+  form do |f|
+    f.inputs "Admin Details" do
+      f.input :email
+      f.input :password
+      f.input :password_confirmation
+    end
+    f.actions
+  end
+
+end