Add the basics of a response handler extension system
Add the basics of a response handler extension system

This stubs in a way to add handlers for processing response body data
in a simple way.

This solution isn't the exact way that a real solution would be done
but sets up the data structures and process flow for how it can be
done:

* there is a list of handlers, they are all processed
* an extension should add to that list
* any extension that is install but can't run for some reason
  should just return
* a handler takes a gabbi test case object

jasonamyers/gabbi,jasonamyers/gabbi