Add invoices API

(Closes #6)

README.md

@@ -43,6 +43,10 @@ called with an error code (if any) and then the response.
    * `.list(count, offset)` - [list customers](https://stripe.com/docs/api#list_customers)
    * `.update_subscription(customer_id, data)` - [update subscription](https://stripe.com/docs/api#update_subscription)
    * `.cancel_subscription(customer_id, at_period_end)` - [cancel subscription](https://stripe.com/docs/api#cancel_subscription)
+* `stripe.invoices` - [Invoices API](https://stripe.com/docs/api#invoices)
+   * `.retrieve(invoice_id)` - [retrieve an existing invoice](https://stripe.com/docs/api?lang=curl#retrieve_invoice)
+   * `.upcoming(customer_id)` - [retrieve the upcoming invoice for a customer](https://stripe.com/docs/api?lang=curl#retrieve_customer_invoice)
+   * `.list(parameters)` - [list invoices](https://stripe.com/docs/api#list_customer_invoices)
 * `stripe.token` - [Tokens API](https://stripe.com/docs/api#tokens)
    * `.create(card_data)` - [create a token](https://stripe.com/docs/api#create_token)
    * `.retrieve(token_id)` - [retrieve a card token](https://stripe.com/docs/api#retrieve_token)

lib/main.js

@@ -144,6 +144,17 @@ module.exports = function (api_key, options) {
                 del("/v1/customers/" + customer_id + '/subscription', { at_period_end: at_period_end }, cb);
             }
         },
+        invoices: {
+            retrieve: function(invoice_id, cb) {
+                get("/v1/invoices/" + invoice_id, {}, cb);
+            },
+            list: function(data, cb) {
+                get("/v1/invoices", data, cb);
+            },
+            upcoming: function(customer_id, cb) {
+                get("/v1/invoices/upcoming", { customer: customer_id }, cb);
+            },
+        },
         token: {
             create: function (data, cb) {
                 post("/v1/tokens", data, cb)