Add MagmaCube interface.

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

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

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