Added a version of getDiscount() for Product that calls the lower-level functions
getLineItemString() for output of the LineItem.
Added test for LineItem.
Fixed GetLineItemString so it calculates the discount correctly.
Added "tests" for LineItem.  Added a new constructor which will be used later when I change the way Receipt creates a LineItem.
Started Receipt class
Added fake tests for int, double, and String data.
I will replace these tests with real functions later.
Added fake tests for int, double, and String data in Customer and FakeDatabase classes.
I will replace these tests with real functions later.
Added accumulators and functions to do the accumulating.