Add convenience function to determine if subscription is 'free' or not

src/appstore.js

@@ -5,6 +5,7 @@ exports = module.exports = {
     unpurchase: unpurchase,
 
     getSubscription: getSubscription,
+    isFreePlan: isFreePlan,
 
     sendAliveStatus: sendAliveStatus,
 
@@ -90,6 +91,10 @@ function getSubscription(callback) {
     });
 }
 
+function isFreePlan(subscription) {
+    return !subscription || subscription.plan.id === 'free';
+}
+
 function purchase(appId, appstoreId, callback) {
     assert.strictEqual(typeof appId, 'string');
     assert.strictEqual(typeof appstoreId, 'string');