Added NSManagedObjectContext+ChainSave method set

Added NSManagedObjectContext+ChainSave method just like [MagicalRecord saveWithBlock…] methods, this set of methods start with the receiver context and creates a child context. Upon saving, the child context saves to parent and chain up the save operation to main context, and finally to saving context. This is to ensure that all changes made in background will pass through to main context and then to persistent store.