Add code climate badge

Hi everyone,

I took the liberty of adding logstash to [code climate](https://codeclimate.com/). It's a service analyzing the code quality of projects checking metrics such as method length, complexity (cyclomatic code complexity I believe),  duplication and others. The service is free for open source.

You can check out the logstash site at code climate [here](https://codeclimate.com/github/elasticsearch/logstash).

Logstash does really well with an overall score of 3.9 out of 4.0 :) The badge I added shows off that score.

As another note, code climate also supports test coverage reporting (regarding issue #2113). We use both over at shoes4, as they seem to have other problems but overall work. 

code climate test coverage is really easy to configure with Travis CI, you seem to be using another (internal) CI server, dunno how easy that is to set up. But that's rather another issue and just a nice to know here.

I signed the CLA.

Cheers,
Tobi