Added paymentMethods resource and listAll method (#214)

* Added paymentMethods resource and listAll method

* Resource case adjustment

lib/mercadopago.js

@@ -2,6 +2,7 @@ var configurations = require('./configurations');
 var backwardSupport = require('./mercadopago-support')();
 var utilsModule = require('./utils');
 var paymentModule = require('./resources/payment');
+var paymentMethodsModule = require('./resources/paymentMethods');
 var preferencesModule = require('./resources/preferences');
 var preapprovalModule = require('./resources/preapproval');
 var merchantOrdersModule = require('./resources/merchantOrders');
@@ -64,6 +65,7 @@ resources = {
   utils: utilsModule,
   configurations: configurations,
   payment: paymentModule,
+  payment_methods: paymentMethodsModule,
   preferences: preferencesModule,
   preapproval: preapprovalModule,
   merchant_orders: merchantOrdersModule,

lib/resources/paymentMethods.js

@@ -0,0 +1,14 @@
+var requestManager = require('../request-manager');
+
+var paymentMethods = module.exports = {}
+
+/**
+ * Get payment methods
+ * [Click here for more infos]{@link https://www.mercadopago.com.br/developers/en/reference/payment_methods/_payment_methods/get/}
+ * @function listAll
+ * @memberof paymentMethods
+ */
+paymentMethods.listAll = requestManager.describe({
+  path: '/v1/payment_methods',
+  method: 'GET'
+});