Marshmallow

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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.

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* Avoid unnecessary attribute access in ``utils.to_marshallable_type`` for improved performance.
* Fix RFC822 formatting for localized datetimes.

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* Can customize validation error messages by passing the ``error`` parameter to a field.
* *Backwards-incompatible*: Rename ``fields.NumberField`` -> ``fields.Number``.
* Add ``fields.Select``. Thanks :user:`ecarreras`.
* Support nesting a Serializer within itself by passing ``"self"`` into ``fields.Nested`` (only up to depth=1).
* *Backwards-incompatible*: No implicit serializing of collections. Must set ``many=True`` if serializing to a list. This ensures that marshmallow handles singular objects correctly, even if they are iterable.
* If Nested field ``only`` parameter is a field name, only return a single value for the nested object (instead of a dict) or a flat list of values.
* Improved performance and stability.