#determine_best_move: plays any of the four available sides
Refactor: make better use of context. Reformat grid arrays.
Create Display class. #game_title displays game title to player
Create Game class. #start displays game title to player
Rename Game class to GameController
Ask human player to choose a mark X or O
Refactor tests: extract into #let rspec helper
Rename game to controller
#set_human_mark sets human player mark for the game
Initialize game controller with a game. #display_board displays current game board