Bonuses

> Adde bonus/malus items \o/ #4

* Add a BonusManager to the game.
* The BonusManager randomly create bonuses until it reaches a cap
  * the popping rate and the capping are adjustable
  * The cap and the rate could eventually be updated by other bonuses
* Bonuses are cleanse at the beginning of the each round
* Turtle & Rabbit bonuses for now

Todo : 
- [x] Find a nice way to modelize which objects are affected by bonus effects (callbacks?)
- [x] Handle collisions with bonuses world wide
  - [x] Handle collisions island wide
- [x] apply effects (sample velocity change test already working)