Add test for swap_players method

At the end of the make_next_move method, the players are going to
swap positions so the game always knows whose turn it is (because it
will always be @players[0]'s turn).  I commented out the tests for
make_human/computer_move so rspec output would be easier to read.
Add swap_players method
refactor make_next_move, add dummy AI to make_human/cpu_move
Uncomment tests for make_next_move, add test for swap_players
Refactoring: add current_player and other_player methods