Jacob branch

This is the expression tree part of the calculator. It has the package to create an expression tree and evaluate it. There's also a procedure to test the expression tree. You can look at the tester function to see how to use the calculator. A few notes:

1.) It assumes (-) means subtraction
2.) It doesn't read parentheses
3.) It doesn't error check (for now)

If you have any questions, feel free to email.