[B] Rename ExplosionPrimeEvent - Fixes BUKKIT-2610, BUKKIT-3885

This renames the ExplosionPrimeEvent to EntityPreExplodeEvent in a non-breaking way.

https://bukkit.atlassian.net/browse/BUKKIT-2610
https://bukkit.atlassian.net/browse/BUKKIT-3885

After this is pulled, BUKKIT-286, BUKKIT-770, and BUKKIT-2231 can continue.

https://bukkit.atlassian.net/browse/BUKKIT-286
https://bukkit.atlassian.net/browse/BUKKIT-770
https://bukkit.atlassian.net/browse/BUKKIT-2231