Allow attributes of associated models to be described in model creation

"Given an item exists with invoice: the invoice, batch.product.name: "Foo 1 100", batch.code: "Bar11", quantity: 10, rate: 82.44" this should be equivalent to
Given a product exists with name: "Foo 1 100" And a batch exists with product: the product, code: "Bar11" And an item exists with batch: the batch, quantity: 10, rate: 82.44

The '.'s in the field name provides the key
This is really useful when creating multiple items
Given the following items exists: | batch.product.name | batch.code | quantity | rate |