task/ta-npi-016-09 -- job search against job_item.item_nbr

nparcel/table/job.py
* new item_nbr_based_job_sql() method which queries job table for
  records that are related to a job_item record with  a given item_nbr

nparcel/table/jobitem.py
* new item_number_sql() method that provides an SQL wrapper to extract
  records where job_item.item_nbr is equal to *item_nbr*