move exceptions from withdraw_balance_to_bank_account to ach_credit
use the same exceptions in charge()
create new module gittip.billing.exchanges
update and move tests
update test fixtures
modify Payday.start() to be a class method
update tests
move constructor to the top
implement creating and capturing a card hold
modify DB self checks to work on cursors
we want to be able to run that check during payday
update comments
add a `stage` column to the `paydays` table
rewrite the payin loop in SQL
after a crash don't rerun the stages that were successfully completed
add some log messages
run a DB check at the end of the payout loop
drop the `pending` column
update tests
improve test coverage of gittip.billing.exchanges