S4u.sqlalchemy

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- Make creation of database tables optional in DatabaseTestCase.

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- Remove bad docstring from DatabaseTestCase class.

- Expose SQLAlchemy engine in test class as `engine` attribute,
and fully dispose of the engine on test teardown.

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- Move dummy test class inside of test class so it does not pollute
metadata of real applications.

- Add an INET type to handle IPv4 and IPv6 networks and addresses.
On the Python side these are handled as IPy.IP instances.

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- Add :py:func:`day_difference <s4u.sqlalchemy.func.day_difference>`
SQL function.

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- Include (very minimal) GUID documentation.

- Add :py:func:`least <s4u.sqlalchemy.func.least>` and
:py:func:`least <s4u.sqlalchemy.func.greatest>` SQL functions.

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- Add basic s4u.upgrade integration.