Add Code Climate badge

As discussed with @stefanpenner, Code Climate is launching JS support very soon and this PR is about providing Ember.js early access to that.

Code Climate is a free-for-OSS tool that helps developers track the quality of their projects over time. We use static analysis to detect possible problem spots in a codebase and then provide feedback so the issues can be reviewed and potentially addressed earlier in the dev cycle. The badge helps make these reports available to contributors.