Added possibility to return a raw HTTP response data

The stub response is initialized with the status code, headers and body of the HTTP response.
The raw HTTP response can be recorded with curl -is for instance.
Added example for returning a raw response with curl -is
Added test usage of the returnRawResponse method in the AFNetworking stubbing spec
Merge pull request #15 from jeanregisser/raw_response

Support for returning recorded raw HTTP responses