[WIP] Bonuses

> Adde bonus/malus items \o/ #4

* Add a bonus stack to the game.
* The game randomly create bonuses until it reaches a cap
  * the popping rate and the capping are adjustable from the game prototype
  * The cap and the rate could eventually be updated by other bonuses
* Bonuses are cleanse at the beginning of the each round

Todo : 
- [ ] Find a nice way to modelize which objects are affected by bonus effects (callbacks?)
- [ ] Handle collisions with bonuses and apply effects

Serialize allow to only send the necessary informations. I tried to send the model "as is" with no luck. How would you do then?

As for the bonuses clearing, could be a performance gain indeed. But events sounds like a more flexible solution overall, also I might need to clear them during a round.