Speed ups and misc fixes

This release has a few speed improvements that especially will speed up things when downloading large sets, and having to restart because the Flickr API inevitably errors out at some point:

• You can now add the --metadata flag to have the script keep track of which files have been downloaded in a way that eliminates the API call to Flickr entirely for an already downloaded image
• The --cache flag now also caches the set list
• One extraneous API call per image download removed

And then a bunch of fixes:

• Actually iterate through all photosets, photos, etc (and not only the first 500)
• Handle error where photo date is too old for the underlying OS to handle

Behind the scenes, there are now a few more tests and everything is type annotated.