Optimize downloading photosets

[beaufour]

Currently when downloading photosets, we do a PhotoSet.getInfo() and a PhotoSet.getPhotos(), and then for each photo a Photo.getInfo() and a Photo.getSizes(). If you call PhotoSet.GetPhotos() with a bunch of extras parameters we'd have just about all the metadata needed to download all the photos, and we could save all the Photo.* calls. It's a bit of a major incision into the flickr API library though as it expects the Photo class to do calls to get its metadata, and the size and save operations needs calls too. But maybe a custom Photo class could work here. It would speed up things quite a bit. Especially as the Flickr API is a bit slow.