Finished refactor of EntityItem death drops. Fixes #996

This completes the refactor which started in commit
MinecraftPortCentral/Cauldron@aadc2f3

* Refactored plugin custom entity support
- Plugins now have the ability to use the CraftWorld.spawnEntity method to
spawn any custom entity. In order for this to work properly, Cauldron will
inject all new EntityTypes into the Bukkit enum. Simply pass the custom
EntityType to CraftWorld.spawnEntity to spawn it.

* Removed most Cauldron custom entity classes as they are no longer used.

* Moved horse inventory dropping which was fixed in commit
Bukkit/CraftBukkit@fb58cc9