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
omg... ok, i understand how it works then. maybe we should change the fixture to "I have a sweep account with a balanced of $100". that way it's more explicit that what the balance is. Or maybe "I have 3 orders for $100 and I those orders for $100 to a sweep account". 

Hmmm, I see your frustration.