Coding style enforcement

This pull request adds a style checker to our project (Rubocop) and configures it according to our coding style.

I've cleaned up all the non-conforming code. There was a lot of it.

`rake test` will now check rubocop unless in ruby 1.8, but it is not in our default `Gemfile` because it only installs on ruby 1.9.3 or greater. It would be nice if there was a way to accomplish a travisci build that didn't involve an extra gemfile, but I couldn't find one.

Since the result is not ruby version dependent, Rubocop will run in just one of our TravisCI build matrix rows. Here's an [example of it passing](https://travis-ci.org/nex3/sass/jobs/11714793).

Each code style change that I made/enforced is in a separate commit for your code reviewing pleasure.