iOS 7 cloud loading procedure.

[UPDATED]   Integrated new iOS 7 cloud loading procedures.
[UPDATED]   Better handle any CFURLCopyResourcePropertiesForKeys errors that aren't caused by the item not existing.
[ADDED]     USMStoreWillChangeNotification before the store is cleared or will change, notifying the application to deactivate its MOCs and reset the UI.
[UPDATED]   URLForCloudContent now returns an NSString* on iOS 7.
[UPDATED]   copyMigrateStore -> migrateStore that takes a strategy argument, allowing migration of different strategies.