Refactored addValidMove() method out of getVerticalMoves(). I will be
able to reuse this method in my other move getting methods.
Wrote getHorizontalMoves(). It was a snap since I had addValidMove().
That was a great idea. Good work John!
Wrote getDiagonalMoves()
Wrote getKnightMoves(). Added a check in addValidMove() to determine if
the posnToCheck is off the board.