Add Board method is_at_opposite_side.

bool is_at_opposite_side(int side, const pos_t &pos);
Add field board_side_ to class Player.
Change Game inner state.

- remove board side from player list;
- implement is_win method via Board method is_at_opposite_side;

Conflicts:
	src/game.cpp
Check if player wins after each move.
End game when one of players wins.
Use smart pointers.
Change board sizes type from size_t to int.