Move saving methods to ManagedObjectContext

this makes more sense and is less "magic".
Add comments to CoreDataKit file
Remove error parameter that wasn't really working
Test CoreDataKit class properties
Document NSManagedObjectContext extension

also removed the concurrencyType from the initializer that creates a contexts that saves directly to the persistent store. Always uses private queue now.
Test NSManagedObjectContext extension methods
Create CoreDataStack to support multiple stacks

for example usable for tests. Multiple stacks are setup, one is the shared and the other stacks are little silos where we can run out tests on.
Fix tests after refactor
Remove duplicate code

setUp code for tests to private func
Private core data stack should not be class scope
Test if saving persists a entity to the context
Make initializers instead of class methods

way nicer :)
Test CoreDataStack
Test NSPersistentStore
Document NSPersistentStoreCoordinator
Test NSPersistentStoreCoordinator
Merge branch 'develop' into feature/importing

Conflicts:
	CoreDataKit.xcodeproj/project.pbxproj
Make importing possible

not yet tested.