delete unuseful code
update text files
fix calculateBuyTwo function
fix calculateHalfPrice function
update text
rename Discount class to PromotionServer class
remove promotion function in Pos class to PromotionServer class
delete unuseful code
move calculate_promotion_test to PromotionServerTest
delete unuseful code
update code
add StorageServer class
add StorageServerTest
success get_items_test
delete unuseful code
success get_cart_barcodes_test
delete Cart Class
success get_promotions_test
delete unuseful function
update code