Added magic method __str__ to the Receipt class
added tests for adding items to a receipt
Made sure all numbers were floats. Added rounding to the str output. Removed total variable from Receipt and added a total() method.
Instead of adding an item to the receipt array, a deepcopy is used instead. That way the Item object can be reused.

Added doc strings to methods.
Added deep copy test
adjusted comments so that they have a maximum line lenght of 72