juju/api/usermanager: add UserInfo

UserInfo was recently added to the server API. This PR adds the client side UserInfo method.
Done. Using juju/errors for all errors.
>
> In state/api/usermanager/client.go:
>
> > +
> > +func (c *Client) UserInfo(username string) (params.UserInfoResult, error) {
> > +	u := params.Entity{Tag: username}
> > +	p := params.Entities{Entities: []params.Entity{u}}
> > +	results := new(params.UserInfoResults)
> > +	err := c.call("UserInfo", p, results)
> > +	if err != nil {
> > +		return params.UserInfoResult{}, err
> > +	}
> > +	if len(results.Results) != 1 {
> > +		//TODO(waigani) not tested
> > +		return params.UserInfoResult{}, fmt.Errorf("expected 1 result, got %d", len(results.Results))
> > +	}
> > +	result := results.Results[0]
> > +	if err := result.Error; err != nil {
> > +		return params.UserInfoResult{}, err
>
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>
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> <https://github.com/juju/juju/pull/91/files#r13788525>.
>