Allow users to specify codeclimate engines

Codeclimate is switching to open source engines http://blog.codeclimate.com/blog/2015/06/19/code-climate-platform/

Als engines conform to a spec https://github.com/codeclimate/spec/blob/master/SPEC.md that has an output that can be parsed.

I propose that when projects have a .codeclimate.yml file such as in https://codeclimate.com/changelog/5653c9b67f7feb0048003051 GitLab will:

1. Add the codeclimate docker container
2. Pass it the contents of the yml file
3. Parse the output and display it nicely in the build results
4. Display the differential of the code quality (module went from A+ to A- in the merge request)

I think it would be handy to have every engine as a separate job in the testing stage.

/cc @ayufan @JobV @dzaporozhets @DouweM