Added interfaces for every entity type. Most (if not all) are empty.

These are place holders at the moment, to be used when determining
entity type. Further functionality could be added to these interfaces
later.

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

@@ -0,0 +1,14 @@
+/**
+ * 
+ */
+package org.bukkit.entity;
+
+/**
+ * Represents a Skeleton.
+ * 
+ * @author Cogito
+ *
+ */
+public interface Skeleton extends Monster {
+
+}