Goparser

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- made library compatible with Python 2.7 and 3.x

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- integrated code more with classes from `genometools` package
- added `GOParser.get_gene_sets()` function (see ``parser.py``)

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- further simplified logging (see ``parser.py``)
- some mostly cosmetic code changes in ``parser.py``

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- simplified the way in which logging is handled (see ``parser.py``)