WorldGen

This is a bugfix release for the following issues:

• Attempting to generate structures on a world marked as "FLAT" would throw exceptions.

Note that although building structures on flat worlds now doesn't crash the plugin, generated items on flat worlds may look strange if there isn't enough land below your construction point (mine shafts, strongholds, and nether fortresses).  You end up with wierd floating structures (side-effect of the base minecraft algorithms).