Add Code Climate badge to README

Hey,

First off, Huginn looks pretty cool. Really interesting approach.

Anyways, I just wanted to drop you a note to let you know that someone added Huginn to Code Climate a while back.

I'm the guy who built Code Climate (automated quality metrics for Ruby) and it's free for OSS. The Huggin code quality reports live here:

https://codeclimate.com/github/cantino/huginn

This PR adds a dynamic badge to the README (similar to Travis) that displays the code quality GPA so that contributors can use the information.

Let me know if you have any questions, or I can help with anything.

Hope you find Code Climate useful.

Best,
Bryan