Add an OP Identifier login button control

[http://blog.nerdbank.net/]

To make buttons like the "Login with Yahoo!" button on the sample RP's login page easier to replicate for multiple OPs, we should have an ASP.NET control that renders as just a hyperlink around an image/text or a button that can redirect the user to an OP using directed identity without any additional call to the RP's server.

Discovery on an OP identifier would happen before the page initially renders (and the results would be cached across page views and sessions to make this really fast) and the result would be a 1-click login that's fast.

[http://blog.nerdbank.net/]

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