Create a policy that will turn any REST call into a JSONP endpoint.

This policy should allow any REST endpoint to be converted into something compatible with JSONP as described (for example) here:

http://en.wikipedia.org/wiki/JSONP

The policy should offer a variety of sensible options to customize the result.

The implementation of the policy will need to manipulate the response payload, something we don't currently have many examples of (but the system is designed to do!).