Make the Reverse Polish Notation calculator work.

We went through the implementation of an RPN calculator in class.
Make it into a complete C++ program and make sure it works properly.

Make sure you use a vector in your implementation.

Here's a small sample of the sort of thing that should work:

1 2 3 4 5 + + + + end
result is 15

1 2 + 3 + 4 + 5 + end
result is 15

1 2 * 3 4 * + 5 + end
result is 19

1 2 + 3 4 + * 5 * end
result is 105

Small extra credit: Make unary operators work.