Rubocop static analysis

Right now, just implements a pretty lean set of rules (i.e. Layout). There is a whole bunch of great lint related rules too, but wanted to focus on a consistent format first. Most IDEs support Rubocop either natively (e.g. RubyMine) or via an extension (e.g. Visual Studio Code). But it can also be run via the command line via bundle exec rubocop lib spec.