juju/api/usermanager: add UserInfo

UserInfo was recently added to the server API. This PR adds the client side UserInfo method.
Done.

> In state/apiserver/usermanager/usermanager_test.go:
>
> > @@ -132,8 +130,8 @@ func (s *userManagerSuite) TestUserInfoUsersExist(c *gc.C) {
> >  	barTag := names.NewUserTag(barfoo)
> >
> >  	userFactory := factory.NewFactory(s.State, c)
>
> Don't need to create your own factory.
>
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> <https://github.com/juju/juju/pull/91/files#r13900971>.
>