Django-sorted-m2m

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* Fix: ``AlterSortedManyToManyField`` operation failed for postgres databases.
* Testing against MySQL databases.

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* Fix: ``AlterSortedManyToManyField`` operation failed for many to many fields
which already contained some data.

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* Fix: When using the sortable admin widget, deselecting an item in the list
had not effect. Thank you to madEng84 for the report and patch!

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* Adding ``AlterSortedManyToManyField`` migration operation that allows you to
migrate from ``ManyToManyField`` to ``SortedManyToManyField`` and vice
versa. Thanks to Joaquín Pérez for the patch!
* Fix: Supporting migrations in Django 1.7.4.
* Fix: The admin widget is not broken anymore for dynamically added inline
forms. Thanks to Rubén Díaz for the patch!

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* Adding support for Django 1.7 migrations. Thanks to Patryk Hes and Richard
Barran for their reports.
* Adding czech translations. Thanks to cuchac for the pull request.

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* Adding support for Django 1.7 and dropping support for Django 1.4.