__construct change

If there is no need of using static initialization:
just a small change in the construct to make the API class usable with dependency injectors such as Pimple and use it for example as a service in a Silex application 
$app['freshbooks'] = $app->share(function () use ($app) {
                return new FreshBooksApi($app['freshbooks_domain'], $app['freshbooks_token']);
            });

the setMethod can be used instead as a substitute to the old construct