Add JSONClient

JSONClient auto-converts Hash <-> JSON in request and response.
* For POST or PUT request, convert Hash body to JSON String with
  'application/json; charset=utf-8' header.
* For response, convert JSON String to Hash when content-type is
  '(application|text)/(x-)?json'

This commit include bin/jsonclient that works as same as bin/httpclient
not with HTTPClient but with JSONClient.