Updated BukkitWorld NMS fields to be static.

This prevents attempts at loading the proper NMS compatibility classes from occurring more than once.

src/main/java/com/sk89q/worldedit/bukkit/BukkitWorld.java

@@ -101,9 +101,9 @@ public class BukkitWorld extends LocalWorld {
 
     private static final Logger logger = WorldEdit.logger;
     private World world;
-    private boolean skipNmsAccess = false;
-    private boolean skipNmsSafeSet = false;
-    private boolean skipNmsValidBlockCheck = false;
+    private static boolean skipNmsAccess = false;
+    private static boolean skipNmsSafeSet = false;
+    private static boolean skipNmsValidBlockCheck = false;
 
     /*
      * holder for the nmsblock class that we should use