[Ur] Added user sign-up and log-in functor to OpenID library

[Adam Chlipala]

I've added some functionality to the OpenID relying party library I 
announced recently:
     http://hg.impredicative.com/openid

The new functionality is a functor that you can apply to app-specific 
settings, such that what comes out takes care of all of the details of 
user sign-up and identity management for your application, using OpenID 
for authentication.

The new module is documented in src/ur/openidUser.urs, and there is a 
small example application in tests/utest.urp.

This code has received minimal testing, and I'm sure there are plenty of 
features worth adding, so I hope folks here are inspired to use it in 
their own apps and report on their experiences!