Made addition work!

The new file, calculatortheory1.py, makes addition work with this
calculator, and sets up the framework for subtraction, multiplication,
and division. This is done without polish notation, much to the chagrin
of Swarthmore CS.
STILL TO DO:
1. revise GUI so it looks presentable (ish)
2. add subtraction, multiplication, and division to the functional
operators list
3. think about adding other operators
4. add a clear button
5. think about using value as a way of carrying an answer over to the
next computation
6. figure out other things that need to be done