Contains:
  - CD.java: a class that represents CD objects
  - CDCollection.java: a class that represents collections of CDs
  - CDGUI.java: a class that represents GUIs for adding/deleting/changing
    CDs in the collection
  - Customer.java: a class that represents Customer objects
  - CustomerCollection.java: a class that represents collections of Customers
  - CustomerGUI.java: a class that represents GUIs for adding/deleting/changing
    Customers in the collection
  - Order.java: a class that represents Order objects
  - OrderLine.java: a class that represents Orde Line objects
  - OrderCollection.java: a class that represents collections of Orders
  - OrderGUI.java: a class that represents GUIs for adding/deleting/changing
    Orders in the collection
  - MainGUI.java: the initial GUI from which the others can be launched
  - ICDGUI.java: an interface that says what it means to be a GUI in this
    program
  - GuiUtility.java: contains a utility method for GUI layout
  - StringUtility.java: contains a utility method for string matching
