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/20151016122334_create_devices.rb

@@ -0,0 +1,12 @@
+class CreateDevices < ActiveRecord::Migration
+  def change
+    create_table :devices do |t|
+      t.string :model
+      t.string :unique_id
+      t.string :imei_number
+      t.string :access_token
+
+      t.timestamps null: false
+    end
+  end
+end