The spectators are not hit by all projectiles (ticket 11)

This is an early Pull-Request. Don't merge now!

- [x] Cancel hits from all projectiles, potions excepted (`EntityDamageByEntityEvent`).
- [ ] Cancel hits from potions (`PotionSplashEvent`).
- [ ] Send a message to the spectator to explain why he has been teleported/teleported back (?).