[FEATURE] Orderbook: Get orderbook for a currency pair (WIP) (RLJS-115)

**Don't merge yet**
I'll look at other parts of the codebase. I know @boxbag was recently refactoring error messaging, so I'll look at the most recent commits he's made.

We use `issuer` in `payments.js` https://github.com/ripple/ripple-rest/blob/develop/api/payments.js#L127 and `orders.js` in the response body: https://github.com/ripple/ripple-rest/blob/develop/test/fixtures/orders.js#L750