Add "VAT Included" setting for orders

1. Create a new In-Commerce Configuration setting, ie. VATIncluded.

2. Add a new [B]VATIncluded[/B] field that will be applied to the Orders table in addition to currently existing - VAT (amount) and VATPercent fields.

3. Based on the Config. setting apply new field to the Order Calculations and Invoicing when the order is place on the Front-end. Show it check-box on "Bill To" step in the Admin and allow changing it.

New field will store the setting for that Order when VAT was Included or Excluded based on described by options below:

[B]Examples how this will be applied:[/B]
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• when prices are displayed without VAT, and user want to have a real
  invoice, we configure VAT (19.6% in my example, actual french rate),
  the result is like this:

Sub-total : 100 $ exc.vat
Shipping : 10 $ exc. vat

VAT 19.6% : 21,56 $ if vat apply to shipping, otherwise VAT
would be 19,60 $

Grand Total : 131,56 $ inc.vat

• when prices are displayed with VAT, and user want to have a real
  invoice :

Sub-total : 100 $ inc.vat
Shipping : 10 $ inc. vat

VAT 19.6% : 18,03 $ Deducted From total price 110 $ if vat apply
to shipping – otherwise VAT is deducted from 100 $ only, VAT total
would be 16.38 $

Grand Total : 110,00 $ inc.vat