Bulk credits sweep account

## Not ready for merge

Initial spec for bulk credits via a sweep account.

### Missing scenarios 

https://github.com/balanced/balanced-api/pull/707#issuecomment-62575260

- [ ] credit several orders into a settlement (credit settlement)
- [ ] credit several orders and reverse a previous credit into a settlement (nett settlement)
- [ ] reverse a previous credit into a settlement (debit settlement)
- [ ] attempt to settle an account with no unsettled transfers
This is the frustrating part of these steps. They're not yet very robust in terms of what you can pass through the many steps it takes to orchestrate an entire situation like this. Right now I know "I have an Account with sufficient funds" creates 3 orders each with a debit of 10000 and credits 10000 from each of the 3 into the customer sweep account.