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
Eh. The settlement should succeed, the following reversal should be pending until another settlement is created to settle up the balance. Then the subsequent GET on the reversal should show it as succeeded. This section has other things wrong with it too. Will fix.