Bug fix in PlayerImpl and updates in QuizGame/Impl

Bug fix in PlayerImpl;
*constructor changed to PlayerImpl (was Player)
*setId changed to this.playerId (was this.id)

QuizGame interface has method addPlayer();

QuizGameImpl updates;
*closeQuiz() method updated to now return the winner and their full
player details as a String
*method getPlayerDetails() added to enable the full player details to
be obtained and returned.