[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.php

@@ -32,7 +32,7 @@ abstract class HTMLPurifier_AttrDef
      * @param $config Mandatory HTMLPurifier_Config object.
      * @param $context Mandatory HTMLPurifier_AttrContext object.
      */
-    abstract public function validate($string, $config, &$context);
+    abstract public function validate($string, $config, $context);
     
     /**
      * Convenience method that parses a string as if it were CDATA.