Added Explosive + TNTPrimed interface

Added Explosive (extends Entity) to Fireball
Added FusedExplosive (extends Explosive) to Creeper, MinecartTNT
Added TNTPrimed (extends FusedExplosive) (Ignite TNTBlock -> TNTPrimed)
My only issue with making a creeper a fused creeper is synchronizing with the client the fuse time. MC Source (core platform for release) does not follow the same logic of a decaying fuse time, rather, a counting up fuse time. Internally, there is a time when the Creeper was activated (ignited if you will), and then the actual time since ignition which counts up comparing to the fuse time.

Whereas a TNT and MinecartTNT counts down in fuse time, creepers don't behave the same way.