Expiring or limited app associations

[aaronshaf]

An entry could be automatically deleted if X months old, or if the application folder has X files.

[RangerMauve]

I think logic like that should be left to applications, though I guess this means we should have timestamps as part of the data to ensure they have that capability.

Ideally, the pub would be as bare-bones as possible so that it would be user easy to implement in other languages and integrate with.