Chef::Config[:file_backup_path] may not exist on brand new Chef nodes

This is a clean re-submit of PR#12 -- https://github.com/cramerdev/tar-cookbook/pull/12.

When download_dir uses specified default "Chef::Config[:file_backup_path]", it does not exist on initial chef-client runs.  In cases where file_backup_path has been altered by client.rb, the parent dirs may not exist.  This fix ensures we have a place to download our remote archive files.