Simple-models

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* Allow None values for non required fields. [Maksim Ekimovskii]
* Code refactoring: moved helpers to utils.py, removed helpers.py, minor

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* Implement ListField, make it json serialize compatible for python2/3; Fixed

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* Initialize callable default values at document init point; Added new
* Implemented ListField, added tests; Forbid mutable default parameters;

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* Implemented choices feature, removed redundant code, updated tests and

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* Implemented ALLOW_EXTRA_FIELDS document attribute; Improve python 3
* Fixed README. [Maksim Ekimovskii]
* Implemented new validators pipeline; Added new immutable field option;
* Validate model defaults; Fixed working with nested models; Added
* Replace SimpleField get_name() with property name. [Maksim Ekimovskii]
* Remove get_instance constructor; Fixed tests. [Maksim Ekimovskii]
* Code refactoring, remove deprecated type parameter from SimpleField;

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* Implemented new feature - optional SimpleField name; Added tests;
* Exception refactoring. [Maksim Ekimovskii]
* Added new field attribute 'error_text' to help to debug validation
* Fixed DictEmbeddedDocument set default functionality + validation;
* Integrate six lib to python 2 to 3 compatibility. [Maksim Ekimovskii]