More robust deletion of SQLite stores
- Core Data makes other supplementary files in addition to the .sqlite file, so deleting those as well for good measure.
- Previously, we had this Core Data deletion implementation in our app code and then MagicalRecord used another Core Data deletion implementation.  It’s best if everyone can use one Core Data deletion implementation.  As a result, making this a public method on NSPersistentStore so that everyone has access to it.