Offline sync via NSPersistentHistory, support for CloudKit Sharing

[deeje]

I really liked your project, and have spent some time adding to it.

• refactored names to standardize on "push" and "pull" to help me better understand the flow of data
• refactored isPublic to support pushing to private, public, or both databases
• incorporated support for relationships via Oleg Müller
• added support for NSPersistentHistoryTracking, to enable offline sync
• added support for CloudKit Sharing via the sharedDatabase
• updated unit tests
• updated the example app

I still need to update the following

• online tests
• documentation
• pass Travis CI but wanted to get this PR up to gather thoughts. Lemme know.

[Sorix]

Thank you a lot for such big PR, I have not a lot of time for that project right now 🙁, so I can't deeply test everything you proposed. I will check if your code is working on a next week, it will outdate all other PRs also.

Do you want a second maintainer status? I can give a push permission for you after code review, so you can push / accept other Pull Requests.

[Sorix]

@deeje, please look at https://github.com/Sorix/CloudCore/pull/51#issuecomment-445068236