Bulk credits sweep account

## Not ready for merge

Initial spec for bulk credits via a sweep account.
> Would the credit say its destination is the sweep_account?

that's implied by the endpoint that you're POSTing to in this scenario.

> If I reverse a credit before it's at the sweep_account, will it immediately go back into the order?

it immediately goes to the sweep, i think you mean bank account. if you reverse a credit it should leave the sweep with a negative balance (if the credit has cleared to the bank account). in that case it would eventually debit the bank account to zero out the balance.