Fix OreGenEvent.Post using the wrong EventBus

OreGenEvent.Post must use MinecraftForge.ORE_GEN_BUS, not
MinecraftForge.EVENT_BUS. Relates to #301.

In addition, the Vein resource is now considered an ore too.
Fix crash when WorldObjects folder doesn't exist

CustomObjects is now responsible for creating the directories.
Don't use empty CustomObjects in client worlds

The client would end up with an empty CustomObjects object, which causes
errors during config loading.