Refactoring - description in extended desctiption

• Changed "\nInvalid move! \n " to Message.InvalidMove and such.
• Renamed Position.cs to PlayerPosition.cs to be more accurate.
• Changed in PlayerPosition.cs X and Y to PositionX and PositionY.
• Added Congratiolations message in Message.cs and some small changes in
it.
• Added StartGame() Method in Labyrinth.cs so that changes do not appear
in the LabyrinthDemo.cs.