Revert "remove htmlpurifier"

This reverts commit c21a462d52.

lib/htmlpurifier/library/HTMLPurifier/ChildDef/Empty.php

@@ -0,0 +1,20 @@
+<?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
+{
+    public $allow_empty = true;
+    public $type = 'empty';
+    public function __construct() {}
+    public function validateChildren($tokens_of_children, $config, $context) {
+        return array();
+    }
+}
+
+// vim: et sw=4 sts=4