OpenID API fixes & usability improvements.

Update usso dependency to support OpenID.
Added a simple example OpenID webapp.
Improvements to common OpenID support:
 - Get OpenID user info from callback with handler function.
 - Fixed callback parameter parsing.
 - Redirects use original scheme, so HTTPS is supported.
 - Improved error handling.
SchemeAuthenticator.Callback now provides User struct and raw OpenID
values to the web application. Updated Ubuntu SSO provider accordingly.
Improved godoc comments.