call print_game_instructions from play, remove mark attr from Human, update specs
update Computer#make_winning_move to check which spots on grid are owned my computer rather than checking for a player mark
update Computer#make_blocking_move to check for spots owned by human
update computer_spec to remove X and O
victory_checker#check_for_win checks for computer's owenship of a row
victory_checker#check_for_win checks for human's  owenship of a row
update VictoryChecker to check for spot ownership rather than specific marks on grid
change board#make_move to mark spot with owner rather than an X or O
update board#print_board to map a player mark to the board depending on who owns a spot
add a test for board#set_board_marks
add computer_mark and human_mark attr to UI, fix bug with creating new_grid
remove unused spec