Dockerise tests & better documentation

[jufemaiz]

Currently trying to make a PR to deal with Postgresql failures due to Arel SQL not providing an alias to :distance and thus returning no values (otherwise I'll just go and use the geocoder gem instead).

Is it possible to adopt Docker as a better solution for testing against multiple databases? This way anyone can more easily contribute without needing to install on their host OS the various databases (and versions of those databases).

Further, it would be good to then amend the README.md to cover off on this.

[mnoack]

I'm a "passive" maintainer @jufemaiz - So I generally accepted any common sense pull request.

However I'm a bit busy now so I can't really review PRs, etc. I'm open to adding people to the project though if you want to help maintain it.