Add SharedCounter class
Add Queue class
Use our portable implementation of multiprocessing.Queue
Add Jean-Baptiste Marquette
Prevent Queue.qsize() from raising NotImplementedError on Mac OS X

multiprocessing.Queue.qsize() may raise NotImplementedError on Unix
platforms like Mac OS X where sem_getvalue() is not implemented. In
order to address this problem, subclass Queue and use an internal
synchronized shared counter whose value is updated when the put() and
get() methods are called. As a side benefit, this makes it possible
to have a reliable version of both qsize() and empty(). Thanks to
Jean-Baptiste Marquette for reporting this problem (#11).

This merges branch 'fix-Queue.qsize-NotImplementedError'