Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'text') and self.text is not None:
            _dict['text'] = self.text
        if hasattr(self, 'tokens') and self.tokens is not None:
            _dict['tokens'] = self.tokens
        if hasattr(self, 'readings') and self.readings is not None:
            _dict['readings'] = self.readings
        if hasattr(self, 'part_of_speech') and self.part_of_speech is not None:
            _dict['part_of_speech'] = self.part_of_speech
        return _dict