ref #121 Public access to native GearmanClient instance

For access to many native client method without write proxy for every native method

Example:
/** @var Mmoreram\GearmanBundle\Service\GearmanClient $gearman */
$gearman = $container->get('gearman');
$gearman->getNativeClient()->setTimeout(100);
$gearman->doNormalJob('AcmeBundleWorkerAcmeWorker~someJob', $params);