Optionally bundle all payments by bank account

Created by: MegaphoneJon

A request from AGBU was to bundle all debit transactions by bank account, as this would greatly simplify bank reconciliation. Given budget concerns, I took this one on myself. I'm submitting the patch to you. Hrant has already tested this and said it's good.

This is intended to go in conjunction with a modification to the Intacct extension to give the option of submitting multiple batches to Intacct per CiviCRM batch - essentially, a separate batch per day even though they will export once/month. Again, pretty specific to their bank rec workflow. I'll look into that when I have a moment.