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
The response should. Most of the steps in these API specs are just written to get the ID so I followed that convention for now, but I'd like to revisit it later.