juju/api/usermanager: add UserInfo

UserInfo was recently added to the server API. This PR adds the client side UserInfo method.
hmm, they were not there last time I looked. I had to massage the time 
zones / locations but all working now. Thanks for pointing them out.

> In state/api/usermanager/client_test.go:
>
> > +
> > +	obtained, err := s.usermanager.UserInfo(tag.String())
> > +	c.Assert(err, gc.IsNil)
> > +	expected := params.UserInfoResult{
> > +		Result: &params.UserInfo{
> > +			Username:       "foobar",
> > +			DisplayName:    "Foo Bar",
> > +			CreatedBy:      "admin",
> > +			DateCreated:    time.Time{},
> > +			LastConnection: time.Time{},
> > +		},
> > +	}
> > +
> > +	// set DateCreated and LastConnection to nil as we cannot know the exact
> > +	// time user was created or last connected.
> > +	// TODO make DateCreated and LastConnection mockable via the user factory.
>
> This is actually easy. MakeUser returns the user that was created and 
> you can access the creation and last connected dates off that.
>
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> <https://github.com/juju/juju/pull/91/files#r13788534>.
>