d-f-s: compute a score for the images, not just the refcount

The idea is that if an image has been used recently, it should have a bigger score than one that was last used 5 weeks ago.

The score multiplier is higher_days_accounted / (last_used_day +1)

For a 5 week range, that means that images used 5 weeks ago will get a multiplier of 1, when images used today will have a multiplier of 35.

Fixes #4 (closed)

Signed-off-by: Benjamin Tissoires benjamin.tissoires@gmail.com