Bulk credits sweep account

### Missing scenarios 

https://github.com/balanced/balanced-api/pull/707#issuecomment-62575260

- [x] credit several orders into a settlement (credit settlement)
- [x] credit several orders and reverse a previous credit into a settlement (nett settlement)
- [x] reverse a previous credit into a settlement (debit settlement)
- [x] attempt to settle an account with no unsettled transfers
- [x] settle account with negative balance from a bank account
- [x] settle account with negative balance from an order with available funds in escrow_amount

### Missing features

- [x] credit.json needs to accept a funding instrument with prefix AT
- [x] marketplaces.json should return links for settlements and accounts