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/API/SuccessResponse.php

@@ -0,0 +1,23 @@
+<?php
+namespace MailPoet\API;
+
+if(!defined('ABSPATH')) exit;
+
+class SuccessResponse extends Response {
+  public $data;
+
+  function __construct($data = array(), $meta = array(), $status = self::STATUS_OK) {
+    parent::__construct($status, $meta);
+    $this->data = $data;
+  }
+
+  function getData() {
+    if(empty($this->data)) {
+      return false;
+    } else {
+      return array(
+        'data' => $this->data
+      );
+    }
+  }
+}