Gapipy

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* Fixed an issue in which modifying a nested dictionary caused gapipy to not
identify a change in the data.
* Added ``tox.ini`` for testing across Python platforms.
* Capture ``403`` Status Codes as a ``None`` object.

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* Provided Python 3 functionality (still Python 2 compatible)
* Removed Python 2 only tests
* Installed ``future`` module for smooth Python 2 to Python 3 migration
* Remove ``DictToModel`` class and the associated tests
* Add ``Dossier`` Resource(s)
* Minor field updates to: ``Customer``, ``InsuranceService``,
``DepartureService``, ``Booking``, ``FlightStatus``, ``State``

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* Fixed a bug with internal ``_get_uri`` function.

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* Adjusted ``Checkin`` resource to meet updated spec.

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* Added ``Checkin`` resource.

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* Fix broken ``Duration`` init in ``ActivityDossier`` (likely broke due to
changes that happened in 2.0.0)