adds benchmark carrier

@csaunders @odorcicd 

I needed a shipping carrier that will sleep for a random amount (0-30s) to simulate the time it takes a real-world 3rd-party API to respond. This will be used by our load-testing tools, and I figured it might be useful to other people using active_shipping.

For more info related to Shopify's use-case see: See: https://github.com/Shopify/shopify/pull/10071#issuecomment-30453485