Improved comment on beginMigration and endMigration.
Replaced property attributeKeys on an entity, with the appropriate Core Data NSEntityDescription methods.
Improved copying of metadata between stores.
Use défaut concurrence type for both contexts in migrator.
Introduced hacks to avoid baseline.zip error.

If you try to load a store too soon after migrating it to the cloud, it
can fail, with an error about a missing baseline.zip file. You can
avoid this by introducing a short delay.
Added some lock/unlocks for PSC.
Changed the saving in migration.
Added a dataset key to the sentinel code

A uuid dataset key has been added. This means if the app switches
between iCloud datasets which both include the current device, it will
be noticed, and handled. Previously this would not be noticed.
Fixed a bug where the dataset id was not being properly added to the user defaults other than on the first machine to sync.
Updated for 10.8 deployment target
Merge pull request #6 from sakrist/master

Updated for 10.8 deployment target
Modernized the source code. Improved migrations, making them asynchronous.