Created verifyPayment alternative function to return Promise (#69)

verifyPayment and cancelSubscription now return promises if no callback is passed

README.md

@@ -33,14 +33,28 @@ var payment = {
 
 ### Purchase verification ( all platforms )
 
-A single method is exposed to verify purchase receipts:
+A method is exposed to verify purchase receipts:
 
 ```javascript
 iap.verifyPayment(platform, payment, function (error, response) {
 	/* your code */
 });
 ```
 
+Or, if you prefer a promise-based alternative: 
+
+```javascript
+iap.verifyPayment(platform, payment)
+.then(
+    response => {	
+        /* your code */ 
+    },
+    error => {
+        /* your code */ 
+    }
+)
+```
+
 The receipt you pass must conform to the requirements of the backend you are verifying with. Read
 the next chapter for more information on the format.
 
@@ -57,6 +71,20 @@ iap.cancelSubscription("google", payment, function (error, response) {
 });
 ```
 
+Or, if you prefer a promise-based alternative: 
+
+```javascript
+iap.cancelSubscription(platform, payment)
+.then(
+    response => {    
+        /* your code */ 
+    },
+    error => {
+        /* your code */ 
+    }
+)
+```
+
 ## Supported platforms
 
 ### Amazon

index.js

@@ -7,8 +7,17 @@ const platforms = {
 	roku: require('./lib/roku')
 };
 
+const promisify = (fn) => {
+	return (...args) => {
+		return new Promise((resolve, reject) => {
+			fn(...args, (err, res) => {
+				return (err ? reject(err) : resolve(res));
+			});
+		});
+	};
+};
 
-exports.verifyPayment = function (platform, payment, cb) {
+function verifyPayment(platform, payment, cb) {
 	function syncError(error) {
 		process.nextTick(function () {
 			cb(error);
@@ -34,10 +43,9 @@ exports.verifyPayment = function (platform, payment, cb) {
 
 		cb(null, result);
 	});
-};
-
+}
 
-exports.cancelSubscription = function (platform, payment, cb) {
+function cancelSubscription(platform, payment, cb) {
 	function syncError(error) {
 		process.nextTick(function () {
 			cb(error);
@@ -66,4 +74,12 @@ exports.cancelSubscription = function (platform, payment, cb) {
 
 		cb(null, result);
 	});
+}
+
+exports.verifyPayment = (platform, payment, cb) => {
+	return (cb ? verifyPayment(platform, payment, cb) : promisify(verifyPayment)(platform, payment));
+};
+
+exports.cancelSubscription = (platform, payment, cb) => {
+	return (cb ? cancelSubscription(platform, payment, cb) : promisify(cancelSubscription)(platform, payment));
 };