Zulu

-------------------

- Make ``Zulu.__sub__`` and ``Zulu.subtract`` return a ``Delta`` object instead of ``datetime.timedelta``.
- Make ``zulu.delta`` and ``Zulu.Delta.parse`` accept a number.
- Allow the first argument to ``Zulu.shift`` be a timedelta or relative delta object.
- Ensure that mathematical magic methods for ``Delta`` return ``Delta`` objects and not ``datetime.timedelta``.

- ``__add__``
- ``__radd__``
- ``__sub__``
- ``__mul__``
- ``__rmul__``
- ``__floordiv__``
- ``__truediv__`` (Python 3 only)
- ``__div__`` (Python 2 only)
- ``__mod__`` (Python 3 only)
- ``__divmod__`` (Python 3 only)

-------------------

- Replace internal implementation of Unicode date pattern formatting with Babel's ``format_datetime``. **breaking change**
- Remove support for formatting to timestamp using ``X`` and ``XX``. **breaking change**
- Rename parse-from-timestamp token from ``X`` to ``timestamp``. **breaking change**
- Add ``zulu.create`` as factory function to create a ``zulu.Zulu`` instance.
- Add locale support to ``Zulu.format`` when using Unicode date pattern format tokens.
- Restore locale support to ``Delta.format``.

-------------------

- Remove locale support from ``Delta.format``. Locale is currently not supported in ``Zulu.format`` so decided to disable it in ``Delta.format`` until both can have it. **breaking change**

-------------------

- Rename ``zulu.DateTime`` to ``zulu.Zulu``. **breaking change**
- Rename ``Zulu.isleap`` to ``Zulu.is_leap_year``. **breaking change**
- Remove ``zulu.format`` alias (function can be accessed at ``zulu.parser.format_datetime``). **breaking change**
- Remove ``Zulu.leapdays``. **breaking change**
- Add ``Zulu.days_in_month``.
- Add ``zulu.Delta`` class that inherits from ``datetime.timedelta``.
- Add ``zulu.delta`` as alias to ``zulu.Delta.parse``.
- Add ``Zulu.time_from``, ``Zulu.time_to``, ``Zulu.time_from_now``, and ``Zulu.time_to_now`` that return "time ago" or "time to" humanized strings.
- Add ``zulu.range`` as alias to ``Zulu.range``.
- Add ``zulu.span_range`` as alias to ``Zulu.span_range``.
- Make time units (years, months, weeks, days, hours, minutes, seconds, microseconds) keyword arguments only for ``Zulu.add/subtract``, but allow positional argument to be an addable/subtractable object (datetime, timedelta, dateutil.relativedelta). **breaking change**

-------------------

- Rename ``DateTime.sub`` to ``DateTime.subtract``. **breaking change**
- Allow the first argument to ``DateTime.add`` to be a ``datetime.timedelta`` or ``dateutil.relativedelta`` object.
- Allow the first argument to ``DateTime.subtract`` to be a ``DateTime``, ``datetime.datetime``, ``datetime.timedelta``, or ``dateutil.relativedelta`` object.
- Provide ``zulu.ISO8601`` and ``zulu.TIMESTAMP`` as parse/format constants that can be used in ``zulu.parse(string, zulu.ISO8601)`` and ``DateTime.format(zulu.ISO8601)``.
- Remove special parse format string ``'timestamp'`` in favor of using just ``'X'`` as defined in ``zulu.TIMESTAMP``. **breaking change**
- Import ``zulu.parser.format`` to ``zulu.format``.
- Fix bug in ``DateTime`` addition operation that resulted in a native ``datetime`` being returned instead of ``DateTime``.

-------------------

- Add ``DateTime.datetime`` property that returns a native datetime.
- Add ``DateTime.fromgmtime`` that creates a ``DateTime`` from a UTC based ``time.struct_time``.
- Add ``DateTime.fromlocaltime`` that creates a ``DateTime`` from a local ``time.struct_time``.
- Add ``DateTime.isleap`` method that returns whether its year is a leap year.
- Add ``DateTime.leapdays`` that calculates the number of leap days between its year and another year.
- Add ``DateTime.start_of/end_of`` and other variants that return the start of end of a time frame:

- ``start/end_of_century``
- ``start/end_of_decade``
- ``start/end_of_year``
- ``start/end_of_month``
- ``start/end_of_day``
- ``start/end_of_hour``
- ``start/end_of_minute``
- ``start/end_of_second``

- Add ``DateTime.span`` that returns the start and end of a time frame.
- Add ``DateTime.span_range`` that returns a range of spans.
- Add ``DateTime.range`` that returns a range of datetimes.
- Add ``DateTime.add`` and ``DateTime.sub`` methods.
- Add ``years`` and ``months`` arguments to ``DateTime.shift/add/sub``.
- Drop support for milliseconds from ``DateTime.shift/add/sub``. **breaking change**
- Make ``DateTime.parse/format`` understand a subset of `Unicode date patterns <http://www.unicode.org/reports/tr35/tr35-19.html#Date_Field_Symbol_Table>`_.
- Set defaults for year (1970), month (1), and day (1) arguments to new ``DateTime`` objects. Creating a new ``DateTime`` now defaults to the start of the POSIX epoch.