juju/api/usermanager: add UserInfo

UserInfo was recently added to the server API. This PR adds the client side UserInfo method.
Thanks for the explanation. I guess that makes sense - this is just differences between how state stores these datetimes and how they're returned by the API.  It would be nice if the API returned DateCreated and LastConnection in the same way though (i.e. both in UTC or both in local). @howbazaar is this an oversight?