when importing, rename jpgs based on time + original md5

this will provide better dupe detection as well as prevent
fusking if images are hosted semi-secretly.
utility to rename files into unique name format (time+md5)

this isn't perfect because the original md5 can't be recreated. but
it's consistent and still unguessable.
don't fall back to ctime for jpg.taken_at. better to know when data is bad.