Django-api-forms

- **Fixed**: `ApiFormException('No clean data provided! Try to call is_valid() first.')` was incorrectly raised if
request payload was empty during `Form::fill` method call
- **Changed**: `clean_data` property is by default `None` instead of empty dictionary

- **Fixed**: Fixed issue with `clean_` methods returning values resolved as False (`False`, `None`, `''`)

- **Fixed**: Ignore `ModelMultipleChoiceField` in `Form::fill()`

One more step to get rid of `pytest` in project (we don't need it)

- **Changed**: Correctly resolve key postfix if `ModelChoiceField` is used in `Form::fill()`
- **Changed**: `DjangoApiFormsConfig` is created

- **Added**: `FileField.content_type` introduced (contains mime)

- **Added**: `FileField` and `ImageField` introduced
- **Added**: Defined extras in `setup.py` for optional `Pillow` and `msgpack` dependencies
- **Added**: Working `Form::fill()` method for primitive data types. Introduced `IgnoreFillMixin`