[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.php

@@ -24,7 +24,7 @@ abstract class HTMLPurifier_Strategy
      * @param $config Configuration options
      * @returns Processed array of token objects.
      */
-    abstract public function execute($tokens, $config, &$context);
+    abstract public function execute($tokens, $config, $context);
     
 }