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 bias is to start off by providing a single way of doing things.  If users ask for a strict validation mode then I'd be happy to add it.  If not, then we've managed to keep our interfaces and documentation just a little bit simpler :)