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

- [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
I don't see a reversals happening in this scenario despite it saying "reverse an account credit".