Added Difficulty API.

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

@@ -34,6 +34,7 @@ import org.bukkit.TreeType;
 import org.bukkit.World;
 import org.bukkit.block.Biome;
 import org.bukkit.generator.BlockPopulator;
+import org.bukkit.Difficulty;
 
 public class CraftWorld implements World {
     private final WorldServer world;
@@ -578,6 +579,14 @@ public class CraftWorld implements World {
         world.savingDisabled = !value;
     }
 
+    public void setDifficulty(Difficulty difficulty) {
+        this.getHandle().difficulty = difficulty.getValue();
+    }
+
+    public Difficulty getDifficulty() {
+        return Difficulty.getByValue(this.getHandle().difficulty);
+    }
+
     public boolean hasStorm() {
         return world.worldData.hasStorm();
     }