Refactor determining rate_value from a reponse node

Refactors fetching the rate value from a USPS response node. Allows flexibility in pulling this out for other systems. Instead of https://github.com/Shopify/active_shipping/pull/192/files.

cc @RichardBlair @jnormore 
It will go in here for internal customization. This simply returns the rate_node as is, with no change to the active_shipping library.