JSON support

In my fork I have added the possibility to fetch data from OpenTSDB in JSON and JSONP by using URLs like

http://127.0.0.1:4242/q?start=1w-ago&m=sum:some-metric&json-data

or

http://127.0.0.1:4242/q?start=1w-ago&m=sum:some-metric&jsonp-data=someCallback

JSON support comes in handy when integrating OpenTSDB into a JavaScript-based application that suffers from the same-domain policy. I'd love to see this merged in the head branch.