Refactoring whos_turn_is_it method to be concise

Simple ternaries are acceptable.  puts statements should
be separate from logic.
Move get_combos to player class

Player should be in charge of its own moves.  This
means that game doesn't have to know about player
moves. It can ask the player.
Make function name more readable

show_gameboard is now display
Better name for player turn

get_move is now names turn
Put low-level board-marking logic into game

created a method in game called mark board that draws
xs and os onto the game board.
Add AI player

AI player class added.  It selects moves randomly.