RED - Testing size()
GREEN - Testing size() is now implemented.
RED - Testing pop().
GREEN - Testing pop() is now implemented and the object is deleted from the stack.
REFACTOR - The pop() method.
RED - Pop() empty should return null.
GREEN - Pop() on empty now returns null.
REFACTOR - The pop() method and removed some test methods from StackTest that was now covered by other tests. Added a  @before method to reduce code in the test methods.