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/`.
`script/test` runs Rubocop and haml-lint in addition to the tests.  When testing to fail, you don't need to run the style checkers. You actually only need to run the tests you wrote, not even the whole suite, but since the suite takes less than 20 seconds to run, it's easier to just say run `spring rspec`. I'll add `to run the tests and style checkers` to the first sentence.