Make the request available to response serializers

I have started work on the rather sizable project of porting RestKit to AFN 2.0. Thus far things have gone reasonably well and, as expected, the serializer architecture is greatly reducing the amount of subclassing and overrides at the seams between the libraries.

One issue that I ran into almost immediately is that the response serializers do not have access to the request that generated the response. I need access to the request for a number of reasons and I can imagine its generally useful, so I’ve amended the serialization protocol and validation methods to include the request.