Add metric_fu and some badges to the README

Nothing fancy for the request.

* added metric_fu as a development dependency
* added a CodeClimate badge
* added a Gemnasium badge
* added a Badge Fury, to show the current related gem version

On a related note, would you have any interesting in setting up Travis CI? I have not used it before, but I think that someone from bazaar labs would need to set this up.