[Ur] Using Persona Authentication API

[Chris Double]

I've been wanting to use the Persona Authentication API [1] instead of
OpenID. I wrote an example based on the Persona Quick Setup [2] and put
it on github:

<https://github.com/doublec/urweb-persona>

For 'real world' usage it'd probably not be too much work to either add
or modify the openid ur/web code to use persona. 

Feedback on the approach I've taken is welcome.

[1] https://login.persona.org/
[2] https://developer.mozilla.org/en-US/Persona/Quick_Setup

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