Some cleanup + edge case / reset handling.

[ADDED]     Some API documentation.
[UPDATED]   Some cleanup.
[FIXED]     Migration safety check should test the right thing, not indirectly.
[UPDATED]   Store reset code consolidated.
[FIXED]     Make sure a new tentative store UUID will be used after resetting the cloud store.
[FIXED]     Save after cloud import to prevent losing changes.  Log errors verbosely.
Some cleanup + edge case / reset handling.

[ADDED]     Some API documentation.
[UPDATED]   Some cleanup.
[FIXED]     Migration safety check should test the right thing, not indirectly.
[UPDATED]   Store reset code consolidated.
[FIXED]     Make sure a new tentative store UUID will be used after resetting the cloud store.
[FIXED]     Save after cloud import to prevent losing changes.  Log errors verbosely.
Update the example to match the new API.
Migration strategies + more robust API + more robust store loading.

[ADDED]     Migration strategies:
                - iOS migration (as before) just uses the PSC's migratePersistentStore.
                - CopyEntities migration attempts to copy all the entities from the source store to the destination and rebuild relationships. (WIP)
                - Manual migration allows the application to do its own migration.
                - None migration does no migration and just loads the existing store or creates an empty one if one doesn't exist.
            This was needed mostly because the iOS migration used before is bugged in iOS 6.1.
[REMOVED]   Direct access to the PSC.  The idea is now that you init your MOC whenever the delegate is triggered for a successful store load.  This means much more reliable and consistent access to the store coordinator and only when the store is ready.
[ADDED]     Delegate method to learn when persistence becomes unavailable (because it's loading a new store).
[ADDED]     Delegate method for dealing with failure to load stores.
[ADDED]     Now the delegate must be set at init time to reliably receive the store load notification that's needed to init your MOC.
[IMPROVED]  The PSC is now locked whenever it is unavailable (store loading) to prevent the application from using it.
[IMPROVED]  Much more robust store loading code.
[IMPROVED]  Better logging and more information in delegate methods.
[ADDED]     Missing nukeCloudContainer which deletes the entire application container from iCloud.