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

**Don't merge yet**
How many different orderbook parameters do you need? I see USD/BTC and BTC/USD. If it's really just two it's probably better to define them both in the fixture. This construct-the-response-inside-of-the-test pattern is probably not a good idea. This pattern has been an unnecessary burden in the past. [Example](https://github.com/ripple/ripple-rest/pull/238/files): expected error changed from tefMAX_LEDGER to tejMaxLedger, and tej-class error response format changed. I had to make this change across tons of constructors like this when the test could have neatly referred to the response definition in a fixture. This is the point of using fixtures.