Define the commerce payment transaction entity type.

Here is a first draft of the commerce_payment_transaction entity type shipped with the commerce_payment module. No forms are defined so far, only database model has been implemented.

Please feel free to blame me :)
Yeah, it definitely makes sense to add price field instead of static fields such as "currency_code" and "amount". So I'm gonna drop theses properties and I'll add a TODO for the price field creation.