Add option to return timestamps as raw filetimes

Originally reported by: **Philippe Lagadec (Bitbucket: [decalage](http://bitbucket.org/decalage), GitHub: [decalage2](http://github.com/decalage2))**

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For now timestamps are returned as datetime objects. For forensics purposes, there should also be a way to extract raw timestamps (either 8-bytes string or 64 bits integer).

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- Bitbucket: https://bitbucket.org/decalage/olefileio_pl/issue/17
