Implemented the end of the game (closes #7)

Should properly support:
- Playing only against NPC (ends when all NPCs are dead)
- Playing against another player (ends when the win condition is
reached. For now it is one of the players has no unit)
- Client disconnections: Will be considered as a forfeit on other
clients
- Server disconnections: Is just a regular disconnections.

Additionnaly, a game can be restarted right away without having to
restart the whole program.