modify private function to public function and test pass
add CalculatorTest .java
add getSubtotal function and test and delete codes
finish getSubtotal function and test
modify getSubtotal function
add getTotalMoney function and test
finish getTotalMoney function and test
modify Calculator.java
finish getSubtotal method and test pass
add  getTotalMoneyAfterPromoting method and test pass
finish getTotalMoneyAfterPromoting method and test pass
add getSavedMoney method and test
finish getSavedMoney method and test
finish printing