Configurable boss sound radii.

Currently, whenever a dragon dies via damage or a wither completes its spawning animation, a loud sound is sent to every player on the server. This PR allows server owners to the radius around which a wither's spawn sound and a dragon's death sound will be heard (two ints; value of 0 or less (default 0) will revert to default behavior). Both settings' defaults emulate vanilla behavior.

These sounds usually freak players out when they occur suddenly and will allow server owners to remove these interruptions from players' server experience.

This PR works by modifying to whom a PacketPlayOutWorldEvent is sent, instead of being every player it is instead the players that should be hearing the sound. Again, not changing the settings will result in unmodified behavior.

(Redo of PR because I screwed the branch up, royally. See https://github.com/SpigotMC/Spigot/pull/133)