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 says that we are going to represent the settlement as a transfer. is this intentional?