[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/AttrDef/CSS/URI.php

@@ -18,7 +18,7 @@ class HTMLPurifier_AttrDef_CSS_URI extends HTMLPurifier_AttrDef_URI
         parent::__construct(true); // always embedded
     }
     
-    public function validate($uri_string, $config, &$context) {
+    public function validate($uri_string, $config, $context) {
         // parse the URI out of the string and then pass it onto
         // the parent object