Fixed merges when saving to root context from background thread

Following up on these previous pull requests:

https://github.com/magicalpanda/MagicalRecord/pull/456
https://github.com/magicalpanda/MagicalRecord/pull/459

The merge is now being scheduled on the main thread using dispatch_async, so by the time any code scheduled post-save (including the completion block for async saves) executes on the main thread, the merge should have taken place.