Pyluach

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* Added `prefix_day` param to ``festival`` and ``holiday`` methods and
functions.

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* Added ``add`` and ``subtract`` methods to ``dates.HebrewDate``.
* Added ``replace`` method to ``CalendarDateMixin``.
* Added missing documentation for `%y` and `%Y` in formatting
``HebrewDate``.

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* Fix subtracting one date from another returning ``float`` instead of ``int``.

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* Fix issue (`24`_) where Shavuos is returned in most months on day 7.

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* Changed equality comparers to compare object identity on unmatched types.
* Equal dates of different types will no longer be considered identical
keys for dicts.
* Added ``strftime`` and ``__format__`` methods to
``dates.GregorianDate``.
* Added ``__format__`` method to ``dates.HebrewDate``.
* Added `withgershayim` parameter to ``dates.HebrewDate.hebrew_day`` and
``dates.HebrewDate.hebrew_year`` methods
* Added ``monthcount`` method to ``hebrewcal.Year``.
* Removed deprecated ``hebrewcal.Month.name`` attribute.
* Implemented HebrewCalendar classes for generating calendars similar to
Calendar classes in the standard library calendar module.

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* Fixed bug in ``hebrewcal.Month`` comparisons when one month is before
Nissan and one is not.