Java Classwork #3 (MarbleBank)
03) For this classwork (and the next few classworks) you will practice creating
    simple standalone classes from which "objects" can be created.  So, for this
    assignment, create a non-static class named 'MarbleBank' in its own file.
    The class should contain two constructors.  The first takes no arguments and
    sets the starting marble count to zero.  The second constructor takes a single
    positive integer as its argument, representing the initial number of marbles
    in the bank, and sets the starting marble count to that number.  For example,
    a call such as

       MarbleBank bank1 = new MarbleBank();

    will create a new MarbleBank object named 'bank1' that will set up a bank
    containing no marbles, while a call such as

       MarbleBank bank2 = new MarbleBank(25);

    will create a new MarbleBank object named 'bank2' that will set up a bank
    containing 25 marbles.  If the argument sent into the second constructor
    for the initial marble count is a negative integer, then the initial marble
    count should be set to zero.

    Note that the above calls to the 'MarbleBank' class are merely examples.  The
    identifiers 'bank1' and 'bank2', as well as the number 25 (or any specific
    marble number), should not appear ANYWHERE in your "MarbleBank.java" file.

    The method 'main' and the keyword 'static' must not appear anywhere in the
    'MarbleBank' class.  Make sure the internal variables in that class are
    private.  The class should contain the following public methods:

       countMarbles
          Arguments:  None
          Displays:   Nothing
          Returns:    An integer equal to the current number of marbles in the
                      bank
       -------------
       addMarbles
          Arguments:  A positive integer representing the number of new marbles
                      to add to the bank
          Actions:    Adds the newly supplied number of marbles to the bank,
                      unless the integer is negative, in which case nothing
                      should happen
          Displays:   Nothing
          Returns:    Nothing
       -------------
       emptyBank
          Arguments:  None
          Actions:    Sets the current number of marbles in the bank to zero
          Displays:   Nothing
          Returns:    Nothing


ADVANCED OPTION FOR CLASSWORK #3
    To the 'MarbleBank' class, add another public method named 'showHistory' that
    displays the number of individual marbles added to the bank during each prior
    call to the 'addMarbles' method since the creation of the 'MarbleBank' object
    or since the last call to 'emptyBank'.  The method should not return anything.