Version 0.19.0

• NEW: Added a retry delegate to allow retries based on GraphQL errors returned from your server, not just network-level errors. NOTE: Be careful with which errors you retry for - the mere presence of an error doesn't necessarily indicate a full failure since GraphQL queries can return partial results. (#770)
• NEW: Automatically generates ApolloEngine/ApolloGraphManager headers based on your main bundle's ID and version number. These can also be configured when you set up your NetworkTransport if you need something more granular for different versions of your application. (#858)
• POSSIBLY BREAKING: The NetworkTransport protocol is now class-bound. If you built your own NetworkTransport implementation instead of one of the ones included with the library, this now must be a class instead of a struct. (#770)
• POSSIBLY BREAKING: Removed an unzip method for arrays of arrays which we were not using. However, since it was public, we figured we should let you know. (#872)
• Bumped Starscream dependency to 3.1.1. (#873)