Add rubocop to project, delivers #179

This adds https://github.com/bbatsov/rubocop to the project with an auto
generated `.rubocop_todo.yml`. The violations can be addressed over time without
breaking the build.
IMHO having it not enforced, you can just do a `gem install rubocop` and run `rubocop`, no need for this PR. See a grape build for example: https://travis-ci.org/intridea/grape/jobs/44609939