task/ta-npe-002-35 -- archive FTP files

nparcel/bin/npftp
* patched the call to the process() method to use the "dry" variable
  rather than "options.dry"

nparcel/conf/npftp.conf
* placeholder for the new "dirs" section and "archive_dir" option

nparcel/exporter.py
* factor out the _create_dir() method to use the generic version in
  nparcel/utils/files.py

nparcel/ftp.py
* support for file archiving
* new method archive_file() to move the Nparcel signature file and
  report to the archive directory