Naming clarification: transaction/payment/transfer
Entries returned by wallet in `get_payments()`, `get_transactions_in()` and `get_transactions_out()` are payments (or transfers), each of them meaning *"Amount X moved from address A to address B"*.

Transactions may contain multiple payments/transfers. Therefore, `Transfer` or `Payment` classes **should not inherit from `Transaction`**. Set membership is not inheritance.

Proposal:

* `Payment` as a base class with `txid` attribute (transaction id/hash) which is a `str()` representation
* `IncomingPayment(Payment)` and `OutgoingPayment(Payment)` returned by wallet, having extra attributes
* `Transaction` as a separate class with `txid` and `blob`. Is returned by `transfer()`, can be submitted to daemon, in future can be retrieved from daemon as well.

TBA