Enhance test and development setup.

- Add and configure Spring gem to speed up testing and development
(https://github.com/rails/spring).

- Create `test` script that will run the specs, along with Rubocop and
haml-lint to check for style issues.

- Update README and CONTRIBUTING to use `script/test` instead of just
`rspec`.

Note: when using `script/test`, haml-lint will not run after Rubocop is
done until we are free of Rubocop offenses. To see the HAML issues
separately, run `haml-lint app/views/`.
I'd keep this kind of comment. It's unlikely to ever become stale unless the gem's core functionality changed and why force someone to have to look up what a gem does if they don't know what it is?