juju/api/usermanager: add UserInfo

UserInfo was recently added to the server API. This PR adds the client side UserInfo method.
oh nice. Updated.

> In state/api/usermanager/client_test.go:
>
> > @@ -72,3 +76,43 @@ func (s *usermanagerSuite) TestCantRemoveAdminUser(c *gc.C) {
> >  	err := s.usermanager.RemoveUser(state.AdminUser)
> >  	c.Assert(err, gc.ErrorMatches, "Failed to remove user: Can't deactivate admin user")
> >  }
> > +
> > +func (s *usermanagerSuite) TestUserInfo(c *gc.C) {
> > +	tag := names.NewUserTag("foobar")
> > +	userFactory := factory.NewFactory(s.State, c)
>
> you don't need this line, a JujuConnSuite has a Factory already.
>
> s.Factory.MakeUser
>
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> <https://github.com/juju/juju/pull/91/files#r13900954>.
>