Create new payment lazily to avoid creating to many transactions.

[pplotka]

The current implementation tries to create new transactions too fast. This is a problem, especially after coming back from PayU. In this case, we don't need to create a new payment. We should retrieve information and only create payment if necessary.

In our business case, we send payment id as extOrderId. When we come back from Payu in the current implementation we got the exception ERROR_ORDER_NOT_UNIQUE.

I think this might be related to #19