Enable inclusive tax calculation

[alko89]

Add option to allow getAmountWithSalesTax to calculate inclusive tax calculation.

Currently getAmountWithSalesTax only adds tax to the price:

  "amount". "100",
  "subtotal": "100",
  "tax": "20",
  "total": "120"

We use inclusive tax calculation, so having this option in the method would be useful:

  "amount". "100",
  "subtotal": "80",
  "tax": "20"
  "total": "100"

I can work on this enhancement if need to.