Sterch-conveyor

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* work around empty results returned by worker's activity

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* got rid of sterch.logfile
* metadirective error rising fixed
* multiple initial stages when the only is defined fixed
* git rid of zope.app.component in the testcases

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* An error in raising ConfigurationError has been fixed (conveyor metadirective)

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* missing files have been added to the distribution

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* regular workers:
- FirstWorker (utilizes only one queue to put initial tasks)
- Worker (utilizes two queues, usually one for input 2nd for output)
- LastWorker (utilizes only one queue to complete final tasks)

* worker groups
- Group --- group of workers (usually it groups workers that performs similar tasks)

* stages --- data processing stages
- FirstStage --- first tasks processing stage (tasks generation)
- Stage --- regular tasks execution
- LastStage --- last task execution

* conveyor --- set of tasks processing stage
- Conveyor

* ZCML directives to register workers

* Test cases