Do not fire EntityDamageEvent for explosions twice. Fixes BUKKIT-XXXX

Explosions directly caused by LivingEntities, such as creepers and tnt lit
by players, have their EntityDamageEvent explicitely handled within
the Explosion class.  In order to prevent double events when damage
is handled for other DamageSources, we need return null for explosion
based damage sources.
Throw event when EnderCrystals harm EnderDragon. Fixes BUKKIT-XXXX

The EntityLiving dealDamage method will not call an event for the
entity damage caused by an explosion without an associated
entity cause, therefore, the explosion caused by EnderCrystals
needs to be explicitely handled within the EnderDragon class.