Moved entity interfaces into parent folder. Fixes #504

src/Entities/UserEntityInterface.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace League\OAuth2\Server\Entities;
+
+interface UserEntityInterface
+{
+    /**
+     * Return the user's identifier.
+     *
+     * @return mixed
+     */
+    public function getIdentifier();
+}