Created stack class

Created and implemented a basic stack class built on top of the
linked list class.

* list.h - created the class header for the stack class.

* list.cpp - implemented the functionality of the stack class by
  wrapping calls to the list class.
Fixed typo in stack header

Fixed a misspelling of the getSize() method in `stack.h`.
Updated makefile

* Makefile - added build instructions for the stack class.
Created stack test code

Created code to test all the public methods of the new stack class.

* main.cpp - added code to test all methods for the stack class.

* list.cpp - removed some debugging code left over from the list
  class tests.