9/19: Added the following statement to the DListNode constructor of one argument.
        previous = null;
     It doesn't change the behavior, because null is the default value, but it
     makes the code more readable to make the initialization explicit.

9/23: The return value for removeFront was not specified.   It should
     be the item removed from the list (or null if the list is empty).

9/25: The comment in the Dlist.java file for insertBack was not
     consistent witthe method name or the README.  (It said to 
     insert at the beginning, when it should have said back.)