* TraditionalCalculator.cs: integration 
   integrated the engine component

* SkunkTest.csproj: added test cases
* MockNumericInputPresenter.cs: added Mock for NumericInputPresenter

* Keypad.cs: changed key layout to match convention

* CalculatorEngine.cs: added error handing for div-zero
fixed an issue where too many args were consumed at once (not
  functional, but it could not be integrated)
added the ability to pop final result


* ICalculator.cs: added interfaces to support error messages and
  results reporting


* MockCalculator.cs: added interfaces

* NumericInputBuffer.cs: fixed a missunderstanding about sign
  presentation

* NumericDisplay.cs: added error handling 

* TestCalculatorEngine.cs: added test cases:
- TestPopResult
- TestDivideByZero


* TestViewModelNumericDisplay.cs: move Mock to its own file
fixed a comment