task/ta22-19 -- business unit IDs as a configurable item

nparcel/config.py
* new dictionary attribute "business_units" which handles the BUs from
  the config
* __call__ method supports "business_units" lookup
* parse_config() method now slurps "business_units"

nparcel/exporter.py
* new method reset() which initialises exporter state to handle
  multiple iterations

nparcel/exporterdaemon.py
* _start() method iterates over each "business_unit" during a pass