API consistency, clarity and cosmetics:
- store/2 took a full dir path whereas read/1 only took the basename;
  now they both take the basename.
- clearly separate the public API and the internal API in exports
- don't have specs for internal API
- remove unused UPGRADE_TABLE define
- ditch rabbit.hrl include - it was only needed for ?MAX_WAIT
- sensible order of function definitions, plus some separators for
  clarity