Initial support for modules SalesOrders and Invoices

Hi.

I'm continuing to extend the library. This time I've added initial support for modules `SalesOrders` and `Invoices`. The specific change that I did is implementing support for nested records that was required to insert an entity from the list above. For now I hardcoded nested tag name to "product" but definitely need to think how to remove the constant and specify it via an argument.

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Maksim Chemerisuk