finish can_get_category_paraments  test
finish can_transform_to_string test
add toString function
move ItemTest CategoryTest and PromotionTest to module package
new Item interface
finish Item interface
create ItemImple class
delete redundant interface function
delete useless codes
implement ItemDao interface
achieve getItemByBarcode
modify the type of returnValue of getItems
achieve getItems
achieve getPromotionOfItem function
achieve getCategory function
rename getPromotionOfItem to getPromotions
create new PromotionDao interface
finish PromotionDao interface
new PromotionImple class
implement PromotionDao interface