Added getClientId method

src/League/OAuth2/Server/Resource.php

@@ -161,6 +161,15 @@ class Resource
         return $this->accessToken;
     }
 
+    /**
+     * Gets the client ID that created the session
+     * @return string
+     */
+    public function getClientId()
+    {
+        return $this->clientId;
+    }
+
     /**
      * Checks if the access token is valid or not.
      *