Added onNotModified method for MKNetworkOperation

Added onNotModified method for MKNetworkOperation, which allows a block to be executed when a server responds with HTTP 304 (not modified). 

This is useful for occasions when we want to refresh some data and have there be some sort of notification when the server has responded, even if it is only with a 304 (e.g., Pull to Refresh). 

The alternative to this would be to have something like completedOperation.isNotModifiedResponse (similar to completedOperation.isCachedResponse).