Added AddValueToRegisterCommand dummy impl.

Added: AddValueToRegisterCommand (0x7XNN) dummy implementation.
Added: NUnitExtensions utilities class with one utility
AssertThrowsArgumentExceptionWithParamName<TArgumentException>.
Change type of operationCode in Command ctor.

Edited: type of operationCode from int? to int in Command constructor.
Edited: type of Byte properties from int? to int in Command with
removing unnecessary checks for null values.
Edited: NullCommand now implements ICommand explicitly instead of
inheriting from Command.