Move NSFetchedResultsController access to iOS-only category

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Remove deprecated +[CoreDataManager instance]

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Simplify CoreDataManager

 - Don't expose helpers that are only used for testing
 - Use TARGET_OS_IPHONE to define differentiating logic

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Use constants when we can

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Make order: method signatures consistent

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Use ObjectiveRelation, not id

Signed-off-by: Stephen Celis <stephen@stephencelis.com>