Postal-address

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* Base our custom ``InvalidAddress`` exception on ``ValueError`` to increase
backward compatibility.

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* Show subdivision as-is in rendered address if not already printed by state,
country or city.
* Document stability policy. Closes 8.
* Change internals to refer to address components as ``fields``.
* Raise custom exception on address validation a detailed list of invalid
fields in one go. Closes 7.

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* Fix bad released version.

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* Add backward compatibility to the ``0.2`` series following territory utils
split out of the ``address`` module. See 8 and 9.

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* Fix rendering of state in address.

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* Remove backward compatibility with the ``0.1`` series.
* Allow direct import of classes from module root.
* Split territory utils out to their own file. See 3.
* Rename ``normalize_country_code`` to ``country_from_subdivision``. Mark the
former as deprecated.
* Address normalization is now non-blocking. It simply reset invalid components
to None. Only a call to ``validate`` method check data consistency.