Possible solution for how to make a projectile not collide with the ground (fixes #15)

* Added a collideWithObstacles flag to ProjectileImpl, used to decide if the projectile should collide with obstacles or not.
* Added a setter for the collideWithObstacles flag to the Projectile interface.
* Made the releaseProjectile method of ProjectileType return the released projectile.
* SpecialProjectileRain now sets the collideWithObstacles flag to false on all the spawned projectiles.

See #15