Added GearmanOutputAwareInterface for worker output

This interface will be checked for when executing workers.
If the worker object implements this interface, then the output of the GearmanExecute is passed
to the worker.

Changes:

* Added the interface Mmoreram\GearmanBundle\Command\Util\GearmanOutputAwareInterface.
* Added GearmanExecute::setOutput().
* Updated GearmanWorkerExecuteCommand to add the output to GearmanExecute.
* Updated GearmanJobExecuteCommand to add the output to GearmanExecute.
* Updated documentation with example worker implementing the interface.