Release v2.0.0

This release includes a major change to the definition
to the Game interface. This change makes it easier to
remove GC pauses during execution, effectively making
this package run twice as fast on games like chess.

Changelog:
-Add Hash() as a required method on Game interface
-Rework action selection system with changes to Game interface
 to consider action indicies instead of slices of actions
  -Replace GetActions() []Action with Len() int
  -Replace ApplyAciton(a Action) with ApplyAction(i int)
-Add error return values for (*MCTS).BestAction()
-Update README to reflect v2.0.0 changes