Make the background context primary and set up inheritance correctly

Derived contexts saves should probably be merged in using the notification system as well.  Right now there is no way to "unregister" a context from the NSNotificationCenter system, hence the explicit call to save the main context.  I don't think this works exactly right.
Make sure to explicitly save the root (background) context when merged
Merge pull request #1483 from wordpress-mobile/issue/1480-core-data-inheritance

Make the background context primary and set up inheritance correctly