Add Remote Flow (#43) and PHP Unit Tests from dansup (#37)

Now the code is following most of PSR
Various changes from various branches (some testing will be required)
Fixed various issues

classes/Activitypub_reject.php

@@ -0,0 +1,58 @@
+<?php
+/**
+ * GNU social - a federating social network
+ *
+ * ActivityPubPlugin implementation for GNU Social
+ *
+ * LICENCE: This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * @category  Plugin
+ * @package   GNUsocial
+ * @author    Diogo Cordeiro <diogo@fc.up.pt>
+ * @author    Daniel Supernault <danielsupernault@gmail.com>
+ * @copyright 2018 Free Software Foundation http://fsf.org
+ * @license   http://www.fsf.org/licensing/licenses/agpl-3.0.html GNU Affero General Public License version 3.0
+ * @link      https://www.gnu.org/software/social/
+ */
+if (!defined('GNUSOCIAL')) {
+    exit(1);
+}
+
+/**
+ * ActivityPub error representation
+ *
+ * @category  Plugin
+ * @package   GNUsocial
+ * @author    Diogo Cordeiro <diogo@fc.up.pt>
+ * @license   http://www.fsf.org/licensing/licenses/agpl-3.0.html GNU Affero General Public License version 3.0
+ * @link      http://www.gnu.org/software/social/
+ */
+class Activitypub_reject extends Managed_DataObject
+{
+    /**
+     * Generates an ActivityPub representation of a Reject
+     *
+     * @author Diogo Cordeiro <diogo@fc.up.pt>
+     * @param array $object
+     * @return pretty array to be used in a response
+     */
+    public static function reject_to_array($object)
+    {
+        $res = array("@context" => "https://www.w3.org/ns/activitystreams",
+                          "type"   => "Reject",
+                          "object" => $object
+                       );
+        return $res;
+    }
+}