Relax validation rules just a little bit.

* Successfully validate dicts that have additional fields beyond those
  specified in the associated model.
* Store raw deserialized JSON in '_raw' attribute of objects.
* Ignore '_raw' attribute in object equality checks.
My understanding is that if the user passes `raw_response=True` then they get very little benefit from using swaggerpy i.e. no response validation, no ability to use any of the models.  It's a shame to give up these feature just to access the decoded JSON representation for a model.