Added API/Endpoint abstract class

- (re)Added Endpoints folder to both API and Router
- fixed syntax in namespaces
- xhr.responseJSON is returned to the fail()
- fixed Router endpoints (view in browser, cron,...)

lib/Router/Endpoints/Queue.php

@@ -0,0 +1,16 @@
+<?php
+namespace MailPoet\Router\Endpoints;
+
+use MailPoet\Cron\Daemon;
+
+if(!defined('ABSPATH')) exit;
+
+class Queue {
+  const ENDPOINT = 'queue';
+  const ACTION_RUN = 'run';
+
+  static function run($data) {
+    $queue = new Daemon($data);
+    $queue->run();
+  }
+}