Update Game.class.php

Some bug fixes and additions.
1. Change path to the current directory of this class file.
2. Player names are passed to the constructor of a game, player1 is white, and player2 is black regardless of names.
3. Return false if the piece is NULL in is_move_legal function, rather than -1.
4. Knight does not need a clear path to move in is_move_legal function.  (needs testing)
5. Created a function legal_moves: returns an array of all currently legal moves.  Could possibly be optimized?