Use RuboCop to enforce default Ruby Style Guidelines.

This adds RuboCop tests to the default test suite, which will enforce a uniform code style.

See:
http://www.joeloliveira.com/2014/02/06/maintain-style-with-rubocop.html

This PR is meant to be a conversation, whether this is desirable to begin with and if any rule changes should be made, etc.

To begin with, I'm going to start on some of the simple ones and ones that can be fixed with --auto-correct.

**NOTE: this should *not be merged* to affect normal contributors until we chip away at the todo.yml significantly.**