Fixes#56, and adds netherbrick rails (for consistency with MTG) if the fences API is found.
Using the MTG fence system adds interop between different fence types, and support for the "Tall fences and walls" option.
I've left the nether's register_craft() call unchanged, it overrides the recipe that default.register_fence() registers, granting 6 fences per crafting instead of MTG's 4.
