If the response to the HTTP request isn't 200 (success), then don't save
the response, and don't call the callback.
Additionally, only retry in the case of HTTPException. This allows using
Ctrl-C to work correctly (and easily).
- add option --quick
check sha1sum of .dirindex files and skip directory if hash matches
- add option --remove-orphan
remove orphan files (files exist locally but not on server)
- be less verbose
- write .dirindex files locally
