Update the /settlements controller to return a SettlementResponse

[sappenin]

When an API client makes a request to /accounts/:id/settlements, the controller should return a JSON object with the amount committed to be settled, and the Settlement Engine's scale, like this:

{
  "amount": "1000",
  "scale": "6"
}

[gakonst]

Rust connector and Ethereum engine implement this in interledger/interledger-rs#199