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/HTMLModule/Tidy/XHTML.php

@@ -0,0 +1,26 @@
+<?php
+
+class HTMLPurifier_HTMLModule_Tidy_XHTML extends HTMLPurifier_HTMLModule_Tidy
+{
+    /**
+     * @type string
+     */
+    public $name = 'Tidy_XHTML';
+
+    /**
+     * @type string
+     */
+    public $defaultLevel = 'medium';
+
+    /**
+     * @return array
+     */
+    public function makeFixes()
+    {
+        $r = array();
+        $r['@lang'] = new HTMLPurifier_AttrTransform_Lang();
+        return $r;
+    }
+}
+
+// vim: et sw=4 sts=4