Declare endpointUrl field

To avoid `field declared dynamically` and `field not found` warnings in IDEs

src/Mailgun/Messages/BatchMessage.php

@@ -40,6 +40,11 @@ class BatchMessage extends MessageBuilder{
      */
 	private $messageIds = array();
 	
+    /**
+     * @var string 
+     */
+	private $endpointUrl;
+
     /**
      * @param \Mailgun\Connection\RestClient $restClient
      * @param string $workingDomain