[Cs3] update templated stack code

[Mikhail Nesterenko]

CS3 students,

As per today's suggestion, I updated the templated stack code here

  http://antares.cs.kent.edu/~mikhail/classes/cs3/Examples/TemplateIntro/Stack/

To use member functions to check if the stack is empty or full before
updating it.

Thanks,
-- 
Mikhail