Completed section 1 of the assignment; added tests for calling
addToEnd(null) - calls like these are now ignored - as well as calling
toString() when the List is empty - returns "[]".
Completed section 2 of the assignment; added methods firstElement,
nextElement, and hasMoreElements, plus tests. NB, as per instructions in
part 2.(c) we assume that nextElement will never be called unless
hasMoreElements returns true.