* App.cs:
* NumericDisplay.cs: added numeric display

* NumericDisplay.cs: added NumericDisplay

* TestViewModelNumericDisplay.cs: test for NumericDisplay's ViewModel

* Calc.csproj: updated with ViewModel and NumericDisplay

* ICalculator.cs: created ITextPresenter to simplify Mock
  implementation

* MockCalculator.cs: modified to reflect changes to interfaces

* NumericEntryMediator.cs: changed variable names

* TraditionalCalculator.cs: updated to report value to NumericDisplay

* SkunkTest.csproj: added test cases for the NumericEntry ViewModel

* TestNumericEntryMediator.cs: cleaned up debug prints