Implement en passant

[Jarkob]

The ability for pawns to take en passant needs to be implemented. For that it needs to be checked if a pawn did not make its first move yet, and if an enemy pawn is standing next to it. For more information see en passant.

[Jarkob]

Difficulty: Is only possible if the black pawn made its move immediately before. The last move needs to be saved.