Valid Biome Check

Added a function to world.hpp which checks to see if a byte is a valid biome.

Will probably also later add a function which gets a descriptive string for a biome value.
Superflat

Added a superflat world generator.

Accepts preset codes in Mojang format, has all the Mojang presets, and is activated by the same world type -- i.e. "FLAT" -- as the Mojang superflat generator.