#16 Replace file-based caching

[Anonymous]

The file-based caching that EnterTrack uses is broken -- when a
cached value changes in the database, the cache is not updated.

I think the best solution would be to replace it with a $_SESSION-based
cache that has a relatively quick timeout -- an hour? We can add a
note to the Administrator's Manual pointing out that users may need to
log out and re-login to see changes to groups (e.g., status, products,
categories).