Added Entity interfaces, changed Player to implement these

By: Dinnerbone <dinnerbone@dinnerbone.com>

paper-api/src/org/bukkit/EntityHuman.java

@@ -0,0 +1,15 @@
+
+package org.bukkit;
+
+/**
+ * Represents a human entity, such as an NPC or a player
+ */
+public interface EntityHuman {
+    /**
+     * Gets the item this entity has currently selected, which will be shown in
+     * their hand
+     *
+     * @return ItemStack containing details on the item this entity has selected
+     */
+    public ItemStack getSelectedItem();
+}