Inventory API Call

Hi Ricbra,

first of all thanks for the awesome library. Helped me a lot to develop an app we use to synchronize the our webshop-inventory with our discogs-inventory.

I know that you generally don`t have any marketplace-API-calls implemented. I assume that it is because they require OAuth. Would you consider supporting the inventory-call? It is an exception in that it is a public call. Please see code changes attached.

Best regards,
David