A callback wrapper that will handle adding things passed to the callback to a 'channel'. A channel is an object that is being passed throughout a callback chain. Each callback may add things to that channel and look up things in that channel.
Given an object, tells you if a path through its keys exists. e.g. Is ['countries', 'US', 'states', 'MA', 'cities', 'Somerville'] in an addresses object?
A callback for `request` that will: 1) Check status codes and call your callback with an error if necessary and 2) pass the body to your callback if everything is OK.