Engine.cs RankList.cs Scores.cs modifications

Score.cs
1) nickName field renamed ot nickname
2) ToString() overriden

RankList.cs
1) default constructor added
2) all methods deleted
3) LoadFromFile(string filePath) method added
4) SaveToFile(string filePath) method added
5) SortScoredDescending() method added

Engine.cs
1) default constructor added
2) AddNewScore is made on 4 separate parts:
- The user is asked for a nickname
- The pair(nickname, score) is added to the rankList
- The whole rankList is saved to a file named "scores.txt"