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/Response.php

@@ -0,0 +1,42 @@
+<?php
+namespace MailPoet\API;
+
+if(!defined('ABSPATH')) exit;
+
+abstract class Response {
+  const STATUS_OK = 200;
+  const STATUS_BAD_REQUEST = 400;
+  const STATUS_UNAUTHORIZED = 401;
+  const STATUS_FORBIDDEN = 403;
+  const STATUS_NOT_FOUND = 404;
+
+  public $status;
+  public $meta;
+
+  function __construct($status, $meta = array()) {
+    $this->status = $status;
+    $this->meta = $meta;
+  }
+
+  function send() {
+    status_header($this->status);
+
+    $data = $this->getData();
+    $response = array();
+
+    if(!empty($this->meta)) {
+      $response['meta'] = $this->meta;
+    }
+    if(!empty($data)) {
+      $response = array_merge($response, $data);
+    }
+
+    if(empty($response)) {
+      die();
+    } else {
+      wp_send_json($response);
+    }
+  }
+
+  abstract function getData();
+}