Django-belt

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* Fixed error in update status inside a handler

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* Added simple validation mixin
* Fixed recursion problem when save in transitions handlers
* Avoid status changes in transitions

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* Added SimpleSerializationMixin
* Added SimplePaginationMixin

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* Removed non used utilities
* Added StatusMixin
* Added SearchQuerySetMixin and SearchFilter
* Added ActionSerializersMixin
* Updated UploadToDir

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* Updated UploadToDir with ``prefix`` param.
* Contrib package with Django Rest Framework utils.

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* Added generators module.
* Added admin widget fot ForeignKeys