add promotion
add getMoney
add BuyTwoGetOnePromotion
add fields
add BuyTwoGetOnePromotionTest
add test should_return_12_when_input_cartItem
override getMoney
add SecondHalfPricePromotion
add SecondHalfPricePromotionTest
test should_return_12_when_input_cartItem
override getMoney
add DiscountPromotionTest
add test should_return_12_when_input_cartItem
overload getMoney
modify getMoney method
refactor DiscountPromotion
modify get method