Fix initialization of biome mappings before plugins load. Fixes #449.

Custom mod biomes mappings were added in CraftBlock's static initializer,
which executes whenever the class is initialized. However this does not
necessarily run before plugins initialize. Change the initializer to
a static method instead, and explicitly invoke it before plugins load,
after all the mods are loaded. As part of this change, move dumpMaterials
to occur after MinecraftServer is initialized.

Fixes mod biome support in plugins using the custom biomes on load,
test plugin in #449.