added game assets

PythonChess/src/ProofOfConcept/black_bishop.png

PythonChess/src/ProofOfConcept/black_horse.png

PythonChess/src/ProofOfConcept/black_king.png

PythonChess/src/ProofOfConcept/black_pawn.png

PythonChess/src/ProofOfConcept/black_queen.png

PythonChess/src/ProofOfConcept/black_rook.png

PythonChess/src/ProofOfConcept/white_bishop.png

PythonChess/src/ProofOfConcept/white_horse.png

PythonChess/src/ProofOfConcept/white_king.png

PythonChess/src/ProofOfConcept/white_pawn.png

PythonChess/src/ProofOfConcept/white_queen.png

PythonChess/src/ProofOfConcept/white_rook.png