minor pawn promotion: give the user choice + check AI

[Daevor]

Currently pawns automatically become queens upon promotion. A user needs to be able to select which piece to promote to (rook, bishop, knight or queen).

Also, I don't know if it currently does, but the AI also needs to consider minor promotions in its calculations.

[wabbott]

In a game I just played, the forced pawn promotion was to a Rook! No choice.

[sylvinus]

thanks for testing out. I'm labelling this as a bug and not a feature because the rules of chess are not optional :) Will fix ASAP.