Add test D4 neighbour positions
Fix tests using AO instead of A1
Add test A1 neighbour positions
Update make to handle new package structure
Add test neighbour_opponent_positions
Extract method Game._is_opponant_position
Extract method neighbour_positions
Remove dead code
Refactor _is_opponant_position
Drop Game.neighbour_positions
Change neighbour related methods to return sets

Changed methods:

  * Game.neighbour_opponent_positions
  * Position.neighbour_positions
Add test left_valid_position when opponant exists
Add test left_valid_position when no opponant exists
Add test top_valid_position when opponant exists
Add test bottom_valid_position when opponant exists
Refactor Position
Refactor Position
Extract method Position.__add__
Refactor Position
Refactor package file layout