add iterator methods

[decalage2]

Originally reported by: Philippe Lagadec (Bitbucket: decalage, GitHub: decalage2)

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add helper methods to iterate dir entries and streams:

• iter_direntries => name, path, direntry
• iter_streams => name, path, direntry
• iter_stream_data => name, path, direntry, reading each stream as a string

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• Bitbucket: https://bitbucket.org/decalage/olefileio_pl/issue/11