clarifies instructions in proxy challenge 

Also added a wikipedia link for complete beginners to read up on proxies and hapi's reply.proxy() ref.
Let me know if this is too much. Thanks.

We should keep it to just the one solution. Choosing between multiple solutions to the same problem can be overwhelming to new learners. If there is a reason to use one over the other, then we can create an additional lesson, or replace this solution with the other.