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/Endpoints/Mailer.php

@@ -0,0 +1,23 @@
+<?php
+namespace MailPoet\API\Endpoints;
+
+if(!defined('ABSPATH')) exit;
+
+class Mailer {
+  function send($data) {
+    $response = array();
+    try {
+      $mailer = new \MailPoet\Mailer\Mailer(
+        (isset($data['mailer'])) ? $data['mailer'] : false,
+        (isset($data['sender'])) ? $data['sender'] : false,
+        (isset($data['reply_to'])) ? $data['reply_to'] : false
+      );
+      $result = $mailer->send($data['newsletter'], $data['subscriber']);
+    } catch(\Exception $e) {
+      $result = false;
+      $response['errors'] = array($e->getMessage());
+    }
+    $response['result'] = ($result) ? true : false;
+    return $response;
+  }
+}