This is multithreaded and will almost probably crash your machine if
you give it a large folder to generate thumbnails for at once.
Also generates a file `pathlist.txt` that contains the publically
accessible URLs for all images that were indexed (For feeding into the
layout generator)
baseuri is the public s3 url
s3_path is where you have your images stored (can be any FS)
browse_path is folders inside your s3_path
thumb_path is where to store the generated thumbnails
thumbsize is how big in pixels the thumbnails should be
