Replace existing autoloader with Composer-supplied

- Move Friendica namespace to `src`
- Move required `ezyang/htmlpurifier` to vendor
- Remove existing static autoloader
- Change boot.php reference to the autoloader

vendor/ezyang/htmlpurifier/library/HTMLPurifier/Language/messages/en-x-test.php

@@ -0,0 +1,11 @@
+<?php
+
+// private language message file for unit testing purposes
+
+$fallback = 'en';
+
+$messages = array(
+    'HTMLPurifier' => 'HTML Purifier X'
+);
+
+// vim: et sw=4 sts=4