Allow instances and singletons (Thanks bmino!!) ((BREAKING))

Allow Instances and Singletons

Author: Jon Eyrick

README.md

@@ -20,15 +20,27 @@ npm install node-binance-api --save
 
 #### Getting started
 ```javascript
-const Binance = require('node-binance-api');
-const binance = new Binance().options({
+const binance = require('node-binance-api').options({
   APIKEY: '<key>',
   APISECRET: '<secret>',
   useServerTime: true, // If you get timestamp errors, synchronize to server time at startup
   test: true // If you want to use sandbox mode where orders are simulated
 });
 ```
 
+#### Instantiating Multiple Instances
+```javascript
+const Binance = require('node-binance-api');
+
+const instance1 = new Binance().options({
+  // ...
+});
+
+const instance2 = new Binance().options({
+  // ...
+});
+```
+
 #### Getting latest price of a symbol
 ```js
 binance.prices('BNBBTC', (error, ticker) => {

node-binance-api.js

@@ -13,9 +13,6 @@
  */
 let api = function Binance() {
     'use strict';
-    if (!(this instanceof Binance)) {
-        return new Binance();
-    }
     const WebSocket = require('ws');
     const request = require('request');
     const crypto = require('crypto');