Minor cleanup and documentation.

- Split out part of _write_json_if_changed() into _is_file_different()
  so that we can check if the metadata has changed without having to
  write it. This let's us avoid writing metadata twice when a set has
  changed. Really minor and insignificant, but I think the code is
  more readable this way.
- Also avoid creating an empty directory then deleting it and recreating
  it for new sets. Really minor, but again I think the code is more
  readable this way.
Fix target of symlinks in sets.

The files in the photostream dir should not have the padded file
number prefix.
Update file hierarchy in README
Store sets under Sets/
Store collections under Collections/
Always write json if the target file does not exist
Only update collections that have changed
Specify directory names once
Remove local sets that have been removed from flickr
Refactor removal of unknown files
Remove local collections that have been removed from flickr