account item-detail should accept an Invoice_Item id as well

[allmightyspiff]

slcli account item-detail expects an Billing_Item id, which is fine. But it should also try to see if the ID provided is an Billing_Invoice_Item id.

Since the invoice-detail command shows an item-id, but is actually a invoice_item id, this could be a bit confusing for users.

Behavior should be as follows for item-detail:

1. Call SoftLayer_Billing_Item::getObject(). (this is what the command currently does already)
2. IF that is a 404 error, try SoftLayer_Billing_Invoice_Item::getBillingItem()