Initial cut at replacing data.json with Cheshire

I don't know if you/anyone is interested, but I have long wanted to modify this library to use the Cheshire JSON library, by the developer of clj-http, which obviously we already use.

I hacked together the following changes.  It needs more testing and thought.
I would like to find a better way to "force" the :as :json directives to clj-http than the way I did here.