Update count_wikipedia.py for Python 3 compatibility and parallel execution

[mhthies]

This commit guarantees full Python 2 and Python 3 compatibility. It also massively improves the performance on multi-processor machines, by using multiprossing.Pool.imap() to delegate the file reading and string processing to sub-processes.

Only drawback in the current version is the missing of the periodic logging of the currently processed file.

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