Refactor Player interface. Add choosable players.

+ Add getPlayerOne() and getPlayerTwo() methods to
  GameController.
+ Add setUp() and LoadPlayer() methods to GameController
  to select human v. computer players.
+ Extract Player interface. Create GamePlayer class.
+ Add LanternaView and LanternaConsole classes for
  graphical terminal views (in progress).
+ Move Player.scoreboard() to MinimaxPlayer class.
  Human players never score boards.
+ Add MinimaxPlayer.scoreMoveWithPruning().
  TODO: Seems slow. Check that branches are actually
        being pruned.