* CalculatorEngine.cs: implemented proper FSM

* TraditionalCalculator.cs: formatting
eliminated operator stack...
added improved class description
one more clean-up...
also fixed a potential race condition
* CalculatorEngine.cs: fixed formatting 

* ICalculator.cs:
* NumericDisplay.cs:
* MockCalculator.cs:
* NumericInputBuffer.cs:
* TraditionalCalculator.cs: fixed formatting