US4: '0' could be '8' case.
US4: Extracted digit strings to separate class.
US4: First test for adding blank, '9' could be '3' case.
US4: Extracted little function.
US4: '8' could be '0'.
US4: '8' could be '9', '8' could be '6'.
US4: Refactoring.
US4: Refactoring removed redundancy.
US4: Added some negative tests 'x' could not be '7'.
US4: Attempt to implement first us4 acceptance test.
US4: Added missing files and extracted OutputPrinter.cs.
US4: Work on first acceptance test.