[TurboGears] Change to IdentityProvider API

[Jeff Watkins]

Starting with Revision 386, IdentityProvider subclasses are no longer required to expose methods for `identity_from_form` and `identity_from_cookie`; instead subclasses are expected to expose `identity_from_request`, which should encompass all methods (including the new `identity_from_http_auth` method). So as far as I recall, the only required methods are `identity_from_request` and `url_for_identity_failure`. And of course, the base IdentityProvider class provides both of these. -- Jeff Watkins http://newburyportion.com/ "Computers are like Old Testament gods; lots of rules and no mercy." -- Joseph Campbell