Fix incorrect concurrency when handling NSManagedObjectDidSaveNotification's.

Apple's docs for NSManagedObjectContextDidSaveNotification state:

	"You can only use the managed objects in this notification on the same thread on which it was posted."

Therefore, -performBlock should not be used to handle these notifications, but performBlockAndWait.

This change removes all errors detected by GDCoreDataConcurrencyDebugging https://github.com/GrahamDennis/GDCoreDataConcurrencyDebugging
Merge pull request #1 from GrahamDennis/concurrency-fix

Fix incorrect concurrency when handling NSManagedObjectDidSaveNotification's