交易完成的通知信

- add gem letter_opener, roadie
- rails g mailer OrderMailer
- add send method notify_order_placed(order) in OrderMail
- add view/order_mailer/notify_order_placed.html.erb
  list order items
  list order total price
  list order info
- add config.action_mailer.delivery_method = :letter_opener in development.rb (for gem letter_opener)
- call OrderMailer.notify_order_placed when order pay