RankListTest.cs + Score.cs modifications

Score.cs
1) Equals(obj) method overriden
2) GetHashCode() metho overriden

RankList.cs
GetTopScoresAsString() access modifier is not now public

RankListTests.cs (more tests added)
public void TestSaveToFile_ShouldSave_ThreeScores()
public void TestSaveToFile_ShouldSave_10Scores()
public void TestGetTopScoresAsString()