Django-dirtyfields

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*New:*

- Adding field comparison method :code:`compare_function` in :code:`DirtyFieldsMixin`.
- Also adding a specific comparison function :code:`timezone_support_compare` to handle different Datetime situations.

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*Bugfix:*

- Not comparing fields that are deferred (:code:`only` method on :code:`QuerySet`).
- Being more tolerant when comparing values that can be on another type than expected.

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*New:*

- Adding :code:`save_dirty_fields` method to save only dirty fields in the database.

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*New:*

- Using :code:`copy` to properly track dirty fields on complex fields.
- Using :code:`py.test` for tests launching.

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*Bugfix:*

- Preventing django db expressions to be evaluated when testing dirty fields (39).

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*New:*

- Using :code:`to_python` to avoid false positives when dealing with model fields that internally convert values (4)

*Bugfix:*

- Using :code:`attname` instead of :code:`name` on fields to avoid massive useless queries on ForeignKey fields (34). For this kind of field, :code:`get_dirty_fields()` is now returning instance id, instead of instance itself.