Adding test case for everything related to setting and getting pieces around the board.

Adding implementation of move for GridBoard.
Adding bounds checking for GridBoard.getPieceAt.

Passing unit tests again! Yay! Happy!
Adding a test where a pawn forks two other pawns.

getLegalMoves should include possible captures.
Note to self: always test for both black and white pieces.

Adding the forgotten implementation of black pawn's capture checks.
Adding null check constraints in Pawn.
Adding legal moves test for white pawns.