task/ta15 -- "job" table update

bin/nparceld.py
* add support for "dry" runs which do not commit to the database

dbsession.py
* set up transaction based DB sessions
* new method rollback() which restores transaction state

loader.py
* add "job" record update functionality
* add support for transaction rollback

table/job.py
* new method update-sql() which returns the SQL update statement for a
  "job" record
* method check_barcode() now supports SQL which returns a list of
  records sorted by date (descending so that most recent is first)