Support for bank accounts in billing (#286)

- Support for bank account in billing options
- Move PaymentMethod and CreditCard functions to Billing
- New PaymentMethodObject that can be either a credit card or bank account
- Move and update unit tests
- Used region to keep JSON properties of PaymentMethod grouped

Author: nwithan8

EasyPost.Tests/BillingTest.cs

@@ -0,0 +1,54 @@
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace EasyPost.Tests
+{
+    [TestClass]
+    public class BillingTest
+    {
+        private TestUtils.VCR _vcr;
+
+        [TestInitialize]
+        public void Initialize()
+        {
+            _vcr = new TestUtils.VCR("billing", TestUtils.ApiKey.Production);
+        }
+
+        [Ignore]
+        [TestMethod]
+        // Skipping due to the lack of an available real payment method in tests.
+        public async Task TestDeletePaymentMethod()
+        {
+            _vcr.SetUpTest("delete_payment_method");
+
+            bool success = await Billing.DeletePaymentMethod(PaymentMethod.Priority.Primary);
+
+            Assert.IsTrue(success);
+        }
+
+        [Ignore]
+        [TestMethod]
+        // Skipping due to the lack of an available real payment method in tests.
+        public async Task TestFundWallet()
+        {
+            _vcr.SetUpTest("fund_wallet");
+
+            bool success = await Billing.FundWallet("2000", PaymentMethod.Priority.Primary);
+
+            Assert.IsTrue(success);
+        }
+
+        [Ignore]
+        [TestMethod]
+        // Skipping due to having to manually add and remove a payment method from the account.
+        public async Task TestRetrievePaymentMethods()
+        {
+            _vcr.SetUpTest("retrieve_payment_methods");
+
+            PaymentMethod paymentMethods = await Billing.RetrievePaymentMethods();
+
+            Assert.IsNotNull(paymentMethods.primary_payment_method);
+            Assert.IsNotNull(paymentMethods.secondary_payment_method);
+        }
+    }
+}