Add TODO.

We should probably be cleaning up local copies of sets and collections
that the user has deleted from their account.
Fix exception when user has no sets.
Document a utility function.
Merge branch 'master' of https://github.com/mattiash/flickrmirrorer into mattiash-master
Minor whitespace fiddling and better TODOs.
Use -q instead of >/dev/null in cronjob command.
Add GitHub user mattiash list of copyright holders
Add --statistics option