Removed unnecessary BasePhoto class

This seemed to be used as a wrapper for flickr.photos.Photo objects. In
practice, it was always immediately unwrapped and turned into a Photo
object via PhotosInterface.getPhoto.
Made logging output more concise

The set and not-in-a-set downloaders now have an info loggin statement
indicating the current photo and progress ("x of y"). To reduce noise, the
downloadUrl() logging statement was demoted to debug level.