World: further populate() testings.
Build: clean cov dir during 'distclean'.
* Implement termite moves (thus the whole game).
* Add axes to Board dump.
Refactoring: rename World to Game.
* Refactoring: rename Board to Game, and move game logic from Board to Game.
* Refactoring: drop singleton design for Game for testing purpose.
Fix: --help and --version do exit with EXIT_SUCCESS.
Update i18n messages.