Fixed catastrophic backtracking in URLValidator.

Thanks João Silva for reporting the problem and Tim Graham for finding the
problematic RE and for review.

This is a security fix; disclosure to follow shortly.

tests/validators/valid_urls.txt

@@ -7,6 +7,7 @@ http://www.example.com/
 http://www.example.com:8000/test
 http://valid-with-hyphens.com/
 http://subdomain.example.com/
+http://a.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 http://200.8.9.10/
 http://200.8.9.10:8000/test
 http://su--b.valid-----hyphens.com/