add subscription methods

lib/charged.js

@@ -131,6 +131,35 @@ Charged.prototype.getSubscriptionTransactions = function(name, callback) {
   return this.get(path, callback, 'transaction');
 };
 
+Charged.prototype.getSubscriptionsByCustomer =
+Charged.prototype.getSubscriptionsByCustomerId = function(customer, callback) {
+  var path = '/customers/' + customer + '/subscriptions';
+  return this.get(path, callback, 'subscription');
+};
+
+Charged.prototype.getSubscriptionsByCustomerRef = function(customer, callback) {
+  var self = this;
+  return this.getCustomerByRef(customer, function (err, customer) {
+    if (err) return callback(err);
+    return self.getSubscriptionsByCustomer(customer.id, callback);
+  });
+};
+
+// http://docs.chargify.com/api-subscriptions#api-usage-json-subscriptions-delayed-cancel
+Charged.prototype.delayedCancelSubscription = function(name, options, callback) {
+  if (!callback) {
+    callback = options;
+    options = { cancel_at_end_of_period: 1 };
+  }
+  var path = '/subscriptions/' + name;
+  return this.put(path, options, callback, 'subscription');
+};
+
+Charged.prototype.resetSubscriptionBalance = function(name, callback) {
+  var path = '/subscriptions/' + name + '/reset_balance';
+  return this.put(path, callback, 'subscription');
+};
+
 /**
  * Statements
  */