Adds one entry point for both JSON and MP APIs

Removes endpoints folder and moves versions to the root
JSON API folder

lib/API/API.php

@@ -0,0 +1,18 @@
+<?php
+namespace MailPoet\API;
+
+if(!defined('ABSPATH')) exit;
+
+class API {
+  static function JSON() {
+    return new \MailPoet\API\JSON\API();
+  }
+
+  static function MP($version) {
+    $api_class = sprintf('%s\MP\%s\API', __NAMESPACE__, $version);
+    if(class_exists($api_class)) {
+      return new $api_class();
+    }
+    throw new \Exception(__('Invalid API version.', 'mailpoet'));
+  }
+}