The idea is that if an object implements __html__ which returns a string this is
used as HTML representation (eg: on escaping). If the object is a str or unicode
subclass and returns itself the object is a safe string type.
This is an updated patch based on jbalogh and ivank patches.
The regex method used until now for the strip_tags utility is fast,
but subject to flaws and security issues. Consensus and good
practice lead use to use a slower but safer method.
