added logic for valid moves

based on the game board, pieces on x coordinates 0 2 4 6 or 8 have the
ability to move up down left right or diagonal, and pieces on x
coordinates 1 3 5 or 7. after checking to make sure a capture move
doesn't exist, isValidMove checks the starting x coordinate and returns
true if it's a valid move and the end coordinate is within the bounds of
the board