Check and checkmate implemented.

After a lot of bugs, I've finally implemented check and checkmate in to my program. So now, a player can't move in to check, a player has to move out of check, and checkmate can be detected, and will end the game.

I also tidied up the code a fair bit, it's very easy for subroutines to get out of hand on a project like this.

And pawn promotion is done. Although you can only promote a pawn to a queen, but it's just not worth implementing pawn promotion to any piece.

One strange little glitch I'm having is when a game is won and the board is reset, a seemingly random piece appears on the square shown in the picture. How peculiar.