Workerpool

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* Python 3 compatibility with six.

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* WorkerPool now inherits from Queue. All of the normal Queue operations apply.
* WorkerPool.wait() has been deprecated in favour of WorkerPool.join().
* WorkerPool constructor now accepts a worker_factory instead of a
(WorkerClass, args) tuple.
* EquippedWorker constructor now accepts a toolbox_factory instead of a
(ToolboxClass, args) tuple.
* Code has been simplified thanks to the previous two changes.
* Added backwards compatibility with Python 2.4, but with reduced
functionality. WorkerPool's join() and task_done() do nothing in Python 2.4
for now.

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* Changed WorkerPool.map() to behave more like Python's built-in map() method.
In consequence, you can no longer define a custom job class to be used by the
WorkerPool.map() method. This will probably change before a 1.0 release.
* Added a unit test for the new WorkerPool.map() functionality.

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* Initial public release.