Simplify ComputerPlayer's turn method

Turn method is more efficient and shorter.
Computer Player checks for two in a row

Computer player checks to see if it has two plays in a row
and plays the third in the row if it is available.

Also, whose turn is it modified and game loop changed from
9.times to an until loop
Test winning_move

Created test for winning move, split game.rb into
two files": one containing classes and one containing
runloop so the classes can be tested in isolation.