juju/api/usermanager: add UserInfo

UserInfo was recently added to the server API. This PR adds the client side UserInfo method.
ugh, fixed now.

> In state/api/usermanager/client_test.go:
>
> > +	c.Assert(err, gc.IsNil)
> > +	expected := params.UserInfoResult{
> > +		Result: &params.UserInfo{
> > +			Username:       "foobar",
> > +			DisplayName:    "Foo Bar",
> > +			CreatedBy:      "admin",
> > +			DateCreated:    user.DateCreated().Local(),
> > +			LastConnection: user.LastConnection().In(obtained.Result.LastConnection.Location()),
> > +		},
> > +	}
> > +
> > +	c.Assert(obtained, gc.DeepEquals, expected)
> > +}
> > +
> > +func (s *usermanagerSuite) TestUserInfoNoResults(c *gc.C) {
> > +	usermanager.Call = func(st *api.State, method string, args, result interface{}) error {
>
> this should be using s.PatchValue otherwise the mock version is still 
> in place after the test!
>
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> <https://github.com/juju/juju/pull/91/files#r13790931>.
>