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* Added new type: URL.
* Added new property to fields: Description.
* Added the optional use os UstraJSON, ultra fast JSON serializer.
* Added 'properties' classmethod to Models. With it, you can get the fields defined in a Model, without instantiate it.
* Improved validation. Now, types derived from basic types has strong checks: int, fload, bool, string, list.
* Improved compatibility with Python 3.