实现了简易的POS机的运算，加入数据库可完善该系统-POS achieve a simple computing machine, add the database can improve the system

简易的超时管理，主要包括上货、盘点、价格管理等，可作为新手学习之用-Simple overtime management, mainly including loading, inventory, price management, etc., can be used as the novice to learn