Convclasses

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* Distribution fix.

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* Removed the undocumented ``Converter.unstruct_strat`` property setter.
* Removed the ability to set the ``Converter.structure_attrs`` instance field.
As an alternative, create a new ``Converter``:

.. code-block:: python

>>> converter = cattr.Converter(unstruct_strat=cattr.UnstructureStrategy.AS_TUPLE)

* Some micro-optimizations were applied; a ``structure(unstructure(obj))`` roundtrip
is now up to 2 times faster.

------------------
* Packaging fixes.
(`17 <https://github.com/Tinche/cattrs/pull/17>`_)

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

* structure/unstructure now supports using functions as well as classes for deciding the appropriate function.
* added `Converter.register_structure_hook_func`, to register a function instead of a class for determining handler func.
* added `Converter.register_unstructure_hook_func`, to register a function instead of a class for determining handler func.
* vendored typing is no longer needed, nor provided.
* Attributes with default values can now be structured if they are missing in the input.
(`15 <https://github.com/Tinche/cattrs/pull/15>`_)
* `Optional` attributes can no longer be structured if they are missing in the input.

In other words, this no longer works:

.. code-block:: python

attr.s
class A:
a: Optional[int] = attr.ib()

>>> cattr.structure({}, A)

* ``cattr.typed`` removed since the functionality is now present in ``attrs`` itself.
Replace instances of ``cattr.typed(type)`` with ``attr.ib(type=type)``.

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

* `Converter.loads` is now `Converter.structure`, and `Converter.dumps` is now `Converter.unstructure`.
* Python 2.7 is supported.
* Moved ``cattr.typing`` to ``cattr.vendor.typing`` to support different vendored versions of typing.py for Python 2 and Python 3.
* Type metadata can be added to ``attrs`` classes using ``cattr.typed``.

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

* Python 3.4 is no longer supported.
* Introduced ``cattr.typing`` for use with Python versions 3.5.2 and 3.6.0.
* Minor changes to work with newer versions of ``typing``.

* Bare Optionals are not supported any more (use ``Optional[Any]``).

* Attempting to load unrecognized classes will result in a ValueError, and a helpful message to register a loads hook.
* Loading ``attrs`` classes is now documented.
* The global converter is now documented.
* ``cattr.loads_attrs_fromtuple`` and ``cattr.loads_attrs_fromdict`` are now exposed.