Add Board#is_token? to help classify tokens, i.e. :x and :o
Add Board#other_token
Use the API provided by Board instead; i.e. Board#is_token? and Board#other_token
Add a test for when there are two winning positions
Replace TTT::Board with TTT:Grid and TTT:Evaluator

  - TTT::Grid represents a 3x3 grid of values
  - TTT::Evaluator has an #analyze method that takes a grid and a player and
    determines the state of the board from the perspective of the player using
    its knowledge of Tic-tac-toe
Update the README
Simplify the implementation of the Tic-tac-toe game engine