refactor ItemDao
refactor ItemDaoImpl
refactor PromotionDao
refactor PromotionDaoImpl
delete useless constructors
modify PromotionFactory
modify PromotionDao
finish PromotionService
add discount promotion case test in PromotionFactoryTest
finish getPromotion method test in PromotionDaoImpl
finish getPromotionBarcode method and test in PromotionServiceImpl
modify ItemDaoImpl
refactor Item.java
refactor ItemServiceImplTest
refactor ItemService
refactor ItemServiceImpl
refactor PromotionServiceImpl
refactor PromotionDiscountTest
refactor PromotionOneFreeTest
refactor PromotionSecondHalf