Implement MKNetworkOperation copyForRetry.

This copies the MKNetworkOperation, with the intention of retrying the call.
This means that the request parameters and callbacks are all preserved, but
anything related to an ongoing request is discarded, so that a new request
with the same configuration can be made.
Merge pull request #332 from tipbit/copyForRetry

Implement MKNetworkOperation copyForRetry.