tdd: StringCalculator Attempt 04: 6 fails
tdd: StringCalculator Attempt 04: 6 passes
tdd: StringCalculator Attempt 04: 6 refactor
tdd: StringCalculator Attempt 04: 7 fails
tdd: StringCalculator Attempt 04: 7 passes
tdd: StringCalculator Attempt 04: 7 refactor
tdd: StringCalculator Attempt 04: 8 fails
tdd: StringCalculator Attempt 04: 8 passes
tdd: StringCalculator Attempt 04: 8 refactor
tdd: StringCalculator Attempt 04: 9 passes/refactor (did not fail)
-closes #4