Some modifications + new tests added

Modifiaction:
SecretNumber.cs
1) cheatResult field renamed and made into a property RevealedReasult
2) default constructor added
3) Cheat() method rename to ReavealDigit() - it no longer displays the
RevealedResult in the console
4) DisplayCheatResult() rename to DisplayRevealedResult() and access
modifier is made public

Engine.cs
1) in ParseGameAction(string command) method, the
this.secretNumber.Cheat() is replaced with

this.secretNumber.RevealDigit();
this.secretNumber.DisplayRevealedResult();

SecretNumberTests.cs (tests added)
public void TestGetGuesses();
public void TestRevealDigit_OneRevealedDigit();
public void TestRevealDigit_AllRevealedDigits();
public void TestGenerateRandomValue();