Put activity list in JSON configuration file
Each activity now polls from a distinct 'taskList' which allows ActivityPoller.php to handle only certain tasks
Tasks to handle by the ActivityPoller are specified as input parameter -c or -j option
New configuration files to pass to the ActivityPoller have been added to the config folder:
 - config/transcoderActivityPoller.json : Contains the config for the Tanscoder activity
 - config/validatorActivityPoller.json : Contains the config for the two Validation activities (ValidateInput and ValidateTranscoded)
WorkflowTracker has been cleanup
ActivityPoller has be deeply remodeled.