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

**Don't merge yet**
Looks like payments and the recent orders code are the odd ones out.

https://gist.github.com/wltsmrz/6e708726ff1d2c4e3df8

Our [schema](https://github.com/ripple/ripple-rest/blob/develop/schemas/Amount.json#L21) suggests `counterparty` also