Add  Server API method: UserInfo

Add a method to the UserManagerAPI which takes a username and returns a struct of user information (username and display name at this stage).
Removed.

> In state/apiserver/usermanager/usermanager.go:
>
> > +			return params.UserInfoResults{}, err
> > +		}
> > +
> > +		user, err := api.state.User(username)
> > +		var result params.UserInfoResult
> > +		if err != nil {
> > +			if errors.IsNotFound(err) {
> > +				result.Error = &params.Error{
> > +					Message: fmt.Sprintf(`No user with the username:"%s" could be found`, username),
> > +					Code:    params.CodeNotFound,
> > +				}
> > +			} else {
> > +				result.Error = common.ServerError(common.ErrPerm)
> > +			}
> > +		} else {
> > +			// TODO(waigani) add ID
>
> I don't think we quite need this comment
>
> —
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> <https://github.com/juju/juju/pull/22/files#r13637184>.
>