Simplify Spreedly Client usage idioms

[mfornos]

Suggestion 1 ^TENTATIVE

Fluent command builder like:

Transaction txn = PurchaseOnGateway.for(client)
    .amount(1234)
    .currency("EUR")
    .retainOnSuccess(true)
    .orderId("XYZ")
    .email("x@x.com")
    .merchantName("Descriptor name")
    .execute(); // here we can override client tokens if needed, or executeAsync();

Instead of this kind of interaction:

String gatewayToken = "XKqtfVWFvZgbwmrN5ZFdMZpB1XN";
String paymentMethodToken = "U6LMHXfN6ZkOPUdXWKx6xO8DydG";

Map<String, String> options = new HashMap<String, String>();
options.put(RETAIN_ON_SUCCESS, "true");
options.put(CURRENCY_CODE, "EUR");
options.put(ORDER_ID, "XYZ");
options.put(EMAIL, "email@example.com");
options.put(MERCHANT_NAME_DESCRIPTOR, "Descriptor name");

Transaction txn = client.purchaseOnGateway(gatewayToken, paymentMethodToken, 1234, options);