adding transaction_id, subscription_id as part of standard response dictionary

[tinio]

Given that the various payment processor gateways have widely varying transaction response objects, it would be incredibly helpful to include 'transaction_id' and 'subscription_id' as part of the standard response dictionary.

As an example, here's the implementation for the authorize.net gateway: tinio/merchant@5595448

@theju, if this sounds agreeable and doesn't go against a possible desire to keep the standard response dictionary simple, I'd be happy to implement this for additional gateways, and submit a proper Pull Request.

Thanks.

[theju]

@tinio, I am fine with your request but certain gateways don't send a transaction_id if the transaction fails. Let me check with all the gateways we have implemented so far.

[tuxcanfly]

Instead of another item in the dictionary, it would be better to have a standard response object, in my opinion. The current response abstraction is leaky - it leaks the backend response class instance.

Will need to go through the responses, filter out the common attributes and consolidate them into a single class. If it's possible at all..