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 :)
I'm pretty sure we should just make payment transactions fieldable and use a price field this time around. I'm not sure it was a very good idea to include the amount / currency_code as payment transaction properties in 1.x.