[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/Strategy/ValidateAttributes.php

@@ -13,7 +13,7 @@ require_once 'HTMLPurifier/AttrValidator.php';
 class HTMLPurifier_Strategy_ValidateAttributes extends HTMLPurifier_Strategy
 {
     
-    public function execute($tokens, $config, &$context) {
+    public function execute($tokens, $config, $context) {
         
         // setup validator
         $validator = new HTMLPurifier_AttrValidator();