task/ta22-20 -- signature file move support for dry run

nparcel/config.py
* new attribute "signature" to hold the Nparcel signature directory
* method parse_config() slurps the signature from the config

nparcel/exporter.py
* method move_signature_file() provides support for "dry" invocations
* rather than skip processing if "staging_dir" is None, continue and
  report on exceptions

nparcel/exporterdaemon.py
* provide configured "signature_dir" as an argument to the "exporter"
  object