Warmup for Tue Oct 14 2014: adding functionality to the Person and Order classes.

Add to the Person class:

1) Add the bank_balance attribute to the Person class
    (just like we did in the other file oo_person_order_classes.py)

2) Add a function / method called can_afford() which takes a number called
    order_total and returns true or false depending on whether the total is
    greater than the Person's  bank_balance
    (just like we did in the other file oo_person_order_classes.py)

Add to the Order class:

3) Add a function / method called add_product() which takes as
    input an Order and adds it to the listOfProducts