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/ChildDef/Empty.php

@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * Definition that disallows all elements.
+ * @warning validateChildren() in this class is actually never called, because
+ *          empty elements are corrected in HTMLPurifier_Strategy_MakeWellFormed
+ *          before child definitions are parsed in earnest by
+ *          HTMLPurifier_Strategy_FixNesting.
+ */
+class HTMLPurifier_ChildDef_Empty extends HTMLPurifier_ChildDef
+{
+    /**
+     * @type bool
+     */
+    public $allow_empty = true;
+
+    /**
+     * @type string
+     */
+    public $type = 'empty';
+
+    public function __construct()
+    {
+    }
+
+    /**
+     * @param HTMLPurifier_Node[] $children
+     * @param HTMLPurifier_Config $config
+     * @param HTMLPurifier_Context $context
+     * @return array
+     */
+    public function validateChildren($children, $config, $context)
+    {
+        return array();
+    }
+}
+
+// vim: et sw=4 sts=4