Added LBFSRetriever super class.
• LBFSRetriever is superclass for all Retriever classes (or will be once we're done refactoring).
• Includes a way for subclasses to have a callback method assigned to cut down on extraneous custom code.
• Affects LBFSCompactListRetriever, LBFSVenuePhotosRetriever, and (new) LBFSVenueHoursRetriever.
• Implementation of changes is complete for LBFSRetriever and LBFSVenuePhotosRetriever.
• Implementation of changes pending for LBFSCompactListRetriever.
• Implementation of LBFSVenuePhotosRetriever pending.
Eliminating unused saved location code.