Marshmallow

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* Add ``Serializer.error_handler`` decorator that registers a custom error handler.
* Add ``Serializer.data_handler`` decorator that registers data post-processing callbacks.
* *Backwards-incompatible*: ``process_data`` method is deprecated. Use the ``data_handler`` decorator instead.
* Fix bug that raised error when passing ``extra`` data together with ``many=True``. Thanks :user:`buttsicles` for reporting.
* If ``required=True`` validation is violated for a given ``Field``, it will raise an error message that is different from the message specified by the ``error`` argument. Thanks :user:`asteinlein`.
* More generic error message raised when required field is missing.
* ``validated`` decorator should only wrap a ``Field`` class's ``output`` method.

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* Fix bug in serializing keyed tuple types, e.g. ``namedtuple`` and ``KeyedTuple``.
* Nested field can load a serializer by its class name as a string. This makes it easier to implement 2-way nesting.
* Make ``Serializer.data`` override-able.

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* Add ``Serializer.factory`` for creating a factory function that returns a Serializer instance.
* ``MarshallingError`` stores its underlying exception as an instance variable. This is useful for inspecting errors.
* ``fields.Select`` is aliased to ``fields.Enum``.
* Add ``fields.__all__`` and ``marshmallow.__all__`` so that the modules can be more easily extended.
* Expose ``Serializer.OPTIONS_CLASS`` as a class variable so that options defaults can be overridden.
* Add ``Serializer.process_data`` hook that allows subclasses to manipulate the final output data.

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* Add ``json_module`` class Meta option.
* Add ``required`` option to fields . Thanks :user:`DeaconDesperado`.
* Tested on Python 3.4 and PyPy.

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* Fix ``Integer`` field default. It is now ``0`` instead of ``0.0``. Thanks :user:`kalasjocke`.
* Add ``context`` param to ``Serializer``. Allows accessing arbitrary objects in ``Function`` and ``Method`` fields.
* ``Function`` and ``Method`` fields raise ``MarshallingError`` if their argument is uncallable.

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* Enable custom field validation via the ``validate`` parameter.
* Add ``utils.from_rfc`` for parsing RFC datestring to Python datetime object.