Add support for retrieval of invoice line items (invoice.lines.all)

According to [Stripe API](https://stripe.com/docs/api#invoice_lines), invoice contains only few line items, to get all of them one should call `invoice.lines.all`.

However calling `invoice.lines.all` in stripe-ruby-mock gives me `Stripe::InvalidRequestError:  No such invoice: test_in_5/lines`