Document API, parameters, and helper methods.

Hey @wpreul, I took a first stab to filling out documentation for this module. I tried to follow conventions found in other hapijs modules, such as [joi](https://github.com/hapijs/joi), but if there's anything that looks out of whack just let me know.

Also, there is some redundancy around the convenience methods, so if you'd rather see that condensed I can make that change.

Finally, the current docs and tests indicate a `downstreamRes` option that doesn't appear to be used anywhere in code, so I did not include that configuration options in this documentation.