Added 'generate-structure' setting support and WorldCreator property.

Fixes BUKKIT-655 and BUKKIT-592

src/main/java/org/bukkit/craftbukkit/CraftWorld.java

@@ -978,4 +978,8 @@ public class CraftWorld implements World {
     public org.bukkit.WorldType getWorldType() {
         return org.bukkit.WorldType.getByName(world.getWorldData().getType().name());
     }
+
+    public boolean canGenerateStructures() {
+        return world.getWorldData().o();
+    }
 }