Django-dirtyfields

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*New:*

- Adding code compatibility for python3,
- Launching travis-ci tests on python3,
- Using :code:`tox` to launch tests on Django 1.5, 1.6, 1.7 and 1.8 versions,
- Updating :code:`runtests.py` test script to run properly on every Django version.

*Bugfix:*

- Catching :code:`Error` when trying to get foreign key object if not existing (32).

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*Bugfix:*

- Removing :code:`model_to_form` to avoid bug when using models that have :code:`editable=False` fields.

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*New:*

- Adding :code:`check_relationship` parameter on :code:`is_dirty` and :code:`get_dirty_field` methods to also check foreign key values.