Yahoo available AX attrs - backchannel/endpoint URLs

[Breno de Medeiros]

> My view here is that anything that is not an explicit API with a
> support SLA can be trusted to last unchanged. Clarification that the
> URL may change will probably not dissuade the RPs that don't have a
> good image hosting solution from deep linking to these images. Serving
> 3rd party images in the domain of the RP's page is actually tricky:
> Just search for GIFAR.
>

I think I messed the double negative above: I meant everything except
explicit APIs with support SLAs are liable to change.

Moreover, much of the other information transmitted via AX is also
liable to change: email address, contact information, so this should
not be treated any differently.