New utilities for migration, PSC locking improvements, iOS 7 fix.

[ADDED]     shouldMigrateFromStoreURL: delegate method which will inform the application when store migration is about to occur.
[ADDED]     setCloudEnabledAndOverwriteCloudWithLocalIfConfirmed: (and vice-versa) to allow an application to enable/disable iCloud but also give the user the option to perform migration of his current data if desired and not implicit.
[UPDATED]   migrateCloudToLocalAndDeleteCloudStoreLocalOnly: -> migrateCloudToLocal.  Essentially, delete is no longer required when migrating cloud -> local.  You can still do it manually by invoking deleteCloudStoreLocalOnly: afterwards.
[ADDED]     Matching migrateLocalToCloud utility.
[IMPROVED]  Locking of the PSC is now more consistent throughout and better enforced.
[FIXED]     iOS 7 appears to have trouble coordinating reading for cloud files, breaking StoreUUIDFile.