New extension methods for saving/updating inventory

[frederikvig]

We need to create some extension methods to make it easy to add/update inventory.

http://world.episerver.com/documentation/developer-guides/commerce/warehouses-and-inventories/Warehouses-and-inventories-examples/

Add a simpler one that doesn't track inventory.

[marisks]

I think that instead of extension methods better would be to create some service class or similar. Extension methods are good for querying but it is not a good practice to use those for an update.