Added constructors to Client and Payment to accept id as parameter

Hi, I've added some constructor to allow a more fluid syntax while building objects for transactions. The new constructors accept the id of the object as parameter (and I had to add also the default constructor because of the override).

For a transaction you usually need Client and Payment objects, adding these constructor allows for a more fluid syntax like:

```
Transaction tx = new Transaction();
tx.setClient(new Client(user.getClientId()));
tx.setPayment(new Payment(user.getPaymentId()));
tx.setAmount(amount);
````
instead of having to create objects and then call the setter methods.

For one-off usage this is easier.