Adding dbox metadata <remote_path>

Hey Ken,

I needed to get the metadata on the directories in staging so I could delete old directories. Here's a pull request for the changes.

Example code using it:

    metadata = Dbox.metadata("staging_book_repos")
    
    to_delete = metadata["contents"].select do |data|
      DateTime.parse(data["modified"]) + 7 < DateTime.now
    end.map do |data|
      data['path']
    end
    
    to_delete.each do |path|
      puts "deleting #{path}"
      Dbox.delete(path)
    end