Some tests added, ScoreEntry code improved.

ScoreEntry.cs
Removed empty constructor.
Name setter optimized, changed exception type from ArgumentNullException
to ArgumentException.
ScoreEntryTests.cs
Added 3 new tests – check behavior with invalid name and mistakes.
IScoreBoard.cs
Added CheckScoreIsHighscore method to the interface.
ScoreBoard.cs
Setter for IsEmpty removed.
GameEngine.cs
Accessing scoreboard through IScoreBooard interface.