an interval triggers all shorter backup intervals

This fixes the slightly buggy behaviour introduced by 81de5bf
and correctly fixes #1

Signed-off-by: Eddy Petrișor <eddy.petrisor@gmail.com>
allow a non-default RSNAPSHOT binary (and easy debugging)

Signed-off-by: Eddy Petrișor <eddy.petrisor@gmail.com>