Added a //generatebiome command, which uses formulae to generate free-form biomes.

Shorthands: //genbiome, //gb
Merge pull request #274 from sk89q/redomasks

Apply masks before history (clone)
Merge pull request #269 from sk89q/generatebiome

Added a //generatebiome command, which uses formulae to generate free-form biomes.
Restored Snapshot API compatibility.