product.rb, 加入數量是否大於庫存的判斷 加入取得庫存量的function
orders_controller.rb, 如果庫存量小於定購數量則重新訂購
views/orders/_form.html.erb, 插入flash[:notice]提示訊息
orders_controller.rb, _form.html.erb, 修改訊息
Merge pull request #35 from garylai1990/feature/fix_stock

Feature/fix stock
Complete flash messages helper