Pull the CoreData header into the PCH to trigger conditional compilation of Core Data specific bits. refs #1126

This is a distateful temporary workaround until I can find a better solution. To try and decouple components from Core Data,
I have introduced conditional compilation against the `_COREDATADEFINES_H` value. This enables you to opt into the Core Data specific
features by adding the import into your target's PCH file. Unfortunately at the moment on CocoaPods, there is no way to add the import
to the PCH only when you pull in the Core Data subspec. I have opened an issue on CocoaPods to pursue a solution.

See https://github.com/CocoaPods/CocoaPods/issues/723