Valideer

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- Added ``ignore_optional_property_errors`` optional parameter in ``parse()`` and
``parsing()``.

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- Added ``returns`` decorator.

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- Python 3 support.

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- Added ``Nullable.default_object_property``.

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- Added ``parsing()`` context manager.
- Added ``Object.REMOVE`` sentinel for removing additional properties.
- Made optional the schema parameter of the ``Range`` validator.
- Fixed docstrings to be compatible with Sphinx.

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- Exposed as top level functions the ``parse``, ``register`` and``register_factory``
Validator static methods (the latter are still kept for backwards compatibility).
- Allow the Nullable default to be a zero-arg callable.
- Added AllOf() composite validator.
- Added ChainOf() composite validator.
- Allow specifying schema of additional object properties or disallowing them,
either locally (by passing an ``additional`` parameter to ``Object``) or
globally (by setting the ``Object.ADDITIONAL_PROPERTIES`` class attribute).
- Added an optional ``additional_properties`` parameter to ``parse()`` to allow
specifying for a single parse call the handling of additional object properties.
- Added an optional ``required_properties`` parameter to ``parse()`` to allow
specifying for a single parse call whether object properties are required or
optional by default. Specifying the same behaviour globally through the
``Object.REQUIRED_PROPERTIES`` attribute is still supported.