Add MagmaCube interface.

Also, add MagmaCube to CreatureType Enum, as per BUKKIT-57. Thanks snowleo!

src/main/java/org/bukkit/entity/CreatureType.java

@@ -27,6 +27,7 @@ public enum CreatureType {
     VILLAGER("Villager", Villager.class),
     BLAZE("Blaze", Blaze.class),
     MUSHROOM_COW("MushroomCow", MushroomCow.class),
+    MAGMA_CUBE("MagmaCube", MagmaCube.class),
     SNOWMAN("Snowman", Snowman.class);
 
     private String name;

src/main/java/org/bukkit/entity/MagmaCube.java

@@ -0,0 +1,7 @@
+package org.bukkit.entity;
+
+/**
+ * Represents a MagmaCube.
+ */
+public interface MagmaCube extends Slime {
+}