Added semi-working magic slabs

Added semi-working magic slabs under package net.bjculk.magic.
The slab functionality is working, but I think crafting for the wool slabs may not work, and each type has one texture. Either dirt, cobblestone, or white wool. At least, this is how I remember it. It's been a few weeks since I touched this code, so it may be in  different shape than I remember.