Time-based snapshots: add script to remove expired snapshots.

After a few days, unless the Valid-Until field was specifically bumped
for a given snapshot, snapshots do expire and need to be cleaned up.
This new script detects snapshots directories for a given archive
directory, checks which snapshots are no longer valid, and removes them
from the reprepro database.

XXX: The actual subdirectory removal hasn't been implemented yet.

This script features a --dry-run flag.

Signed-off-by: Cyril Brulebois <cyril@debamax.com>