Specify explicitly which price includes tax and which not

[PetrDlouhy]

There are few places, where the price or it's tax is set for the payment:

• Payment.total
• Payment.tax
• PurchasedItem.price

It is not entirely clear from the docs, if the prices includes tax or not and if the tax is tax amount or tax rate.
From the implementation of PayPal provider, it is clear, that both prices should be without tax.

I would like to prevent situation, where every provider has it's own interpretation of the values and thus they are incompatible.

I also realised, that the provided information is not enough for my PayU provider, because PayU requires prices for the items to include the tax. I can also imagine provider, that requires tax rate to be stated for every item separately. This problem I solve in PR #224