Implement Assignment #2 Required Task 4, Undo button.

When user is in middle of entering number, Undo acts like backspace.
Once all digits have been removed, result from current program is displayed.
When user is not in middle of entering number, Undo removes top item from program stack and updates UI.
Refactoring to reduce LOC.
Use segmented control to provide different variables values.
Set UIBarStyle to Default.
Update .gitignore with xcuserdata/.