Comparator

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- DEPRECATED - the ``query_df`` method on ``BaseDb`` and subclasses
- DEPRECATED - the ``output`` kwarg for Comparator results
- adds the ``execute`` method on ``BaseDb`` and subclasses
- adds the ``QueryResult`` and ``QueryResultRow`` classes
- adds the ``ComparatorSet`` class
- adds ``list_tables`` and ``delete_table`` methods to ``BigQueryDb``
- cleans up some python 2/3 compatability using six

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- officially support Python 2.7, 3.6, and 3.7

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- adds ``query_df`` methods for returning pandas DataFrames
- adds ``output`` kwarg to Comparator to allow calling the ``query_df`` method

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- initial release