Adds Gemfile

Adds [`Gemfile`](http://bundler.io/v1.3/gemfile.html) for simple [Ruby Gem](https://rubygems.org/) dependency installation, as discussed in #508.

Also updates `README.md` and `src/README.md`, reducing the Jekyll, Sass and Kramdown installation steps to `bundle install`.

Depends on pull request #514.

I suggest we remove all installation steps from `README.md` and expand `src/README.md`, so there's only one place with the instructions. Would a PR for this be welcome?