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/migrate/20151016121931_create_active_admin_comments.rb

@@ -0,0 +1,19 @@
+class CreateActiveAdminComments < ActiveRecord::Migration
+  def self.up
+    create_table :active_admin_comments do |t|
+      t.string :namespace
+      t.text   :body
+      t.string :resource_id,   null: false
+      t.string :resource_type, null: false
+      t.references :author, polymorphic: true
+      t.timestamps
+    end
+    add_index :active_admin_comments, [:namespace]
+    add_index :active_admin_comments, [:author_type, :author_id]
+    add_index :active_admin_comments, [:resource_type, :resource_id]
+  end
+
+  def self.down
+    drop_table :active_admin_comments
+  end
+end