[3.0.0] Convert all $context calls away from references

- Update TODO list
- URISchemeRegistry doesn't return a reference for instance anymore, should do the same for other singletons

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1477 48356398-32a2-884e-a903-53898d9a118a

library/HTMLPurifier/ChildDef/Empty.php

@@ -14,7 +14,7 @@ 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) {
+    public function validateChildren($tokens_of_children, $config, $context) {
         return array();
     }
 }