RMStoreUserDefaultsPersistence has now a method for getting the NSUserDefaults

This allows subclasses of RMStoreUserDefaultsPersistance to override this method and return a different NSUserDefaults object e.g.
```objc
-(NSUserDefaults *)
    return [[NSUserDefault alloc] initWithSuiteName:@"group.company.app"];
}
```

This is needed for extensions like iOS 8 widgets.