Fix possible NSRangeException in -tableView:canEditRowAtIndexPath:

On iOS 8 NSRangeException was being thrown after calling -removeItem: and then -reloadSectionWithAnimation:
The fix adds checks if section and row indices are within bounds.
Merge pull request #179 from fastred/fix_nsrangeexception

Fix possible NSRangeException in -tableView:canEditRowAtIndexPath: