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/Track.php

@@ -0,0 +1,23 @@
+<?php
+namespace MailPoet\Router\Endpoints;
+
+use MailPoet\Statistics\Track\Clicks;
+use MailPoet\Statistics\Track\Opens;
+
+if(!defined('ABSPATH')) exit;
+
+class Track {
+  const ENDPOINT = 'track';
+  const ACTION_CLICK = 'click';
+  const ACTION_OPEN = 'open';
+
+  static function click($data) {
+    $clicks = new Clicks($data);
+    $clicks->track();
+  }
+
+  static function open($data) {
+    $opens = new Opens($data);
+    $opens->track();
+  }
+}