Zulu

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- Add ``zulu.parse_delta`` as alias for ``Delta.parse``.
- Deprecate ``zulu.delta`` in favor of ``zulu.parse_delta``.
- Allow first argument to ``Zulu()``, ``Zulu.parse()``, and ``zulu.parse()`` to be a ``dict`` containing keys corresponding to initialization parameters.
- Fix error message for invalid timezone strings so that the supplied string is shown correctly.

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- Require ``python-dateutil>=2.6.0``. (**breaking change**)
- Replace usage of ``pytz`` timezone handling for strings with ``dateutil.tz.gettz``. Continue to support ``pytz`` timezones during ``Zulu()`` object creation. (**breaking change**).
- Replace default UTC timezone with ``dateutil.tz.tzutc()``. Was previously ``pytz.UTC``. (**breaking change**)
- Replace local timezone with ``dateutil.tz.tzlocal()``. Was previously set by the ``tzlocal`` library. (**breaking change**)

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- Add comparison methods to ``Zulu``:

- ``is_before``
- ``is_on_or_before``
- ``is_after``
- ``is_on_or_after``
- ``is_between``

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- Optimize ``Zulu()`` constructor by eliminating multiple unnecessary calls to ``datetime`` constructor.

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- Fix ``Zulu`` not being pickle-able.

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- Add missing magic method overrides for ``Delta`` for ``+delta``, ``-delta``, and ``abs(delta)`` so that ``Delta`` is returned instead of ``datetime.timedelta``.

- ``__pos__``
- ``__neg__``
- ``__abs__``