Add request(), hasNextPage(), and requestNextPage() (#887)

* Add support for all rippled APIs, including subscriptions.
* Add support for arbitrary stream message types.
* Note that rippled APIs take amounts in drops.
* request() will be available in ripple-lib version 1.0.0+

src/ledger/ledger.ts

@@ -15,7 +15,7 @@ async function getLedger(
   // 1. Validate
   validate.getLedger({options})
   // 2. Make Request
-  const response = await this._request('ledger', {
+  const response = await this.request('ledger', {
     ledger_index: options.ledgerVersion || 'validated',
     expand: options.includeAllData,
     transactions: options.includeTransactions,