Django-api-forms

- **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`

- **Added**: `BaseForm._request` property introduced (now it's possible to use request in `clean_` methods)

- **Fixed**: Fixed `Content-Type` handling if `charset` or `boundary` is present

- **Fixed**: Do not call resolvers methods, if property is not required and not present in request

- **Changed**: Non specified non-required fields will no longer be available in the cleaned_data form attribute.