Add live tests

In order to verify actual API usage, FakeMediaWiki is not sufficient. Testing against live MediaWiki installations provided by Docker should give a lot more confidence and even allows to easily test different versions.

Requires [Docker](https://docker.com) and [mediawiki-testwiki](https://rubygems.org/gems/mediawiki-testwiki); live tests are skipped when `mediawiki-testwiki` is not available. Docker images for various MediaWiki versions are available at the [Docker Registry](https://registry.hub.docker.com/u/blackwinter/mediawiki-testwiki).

Status:

* Most tests from `gateway_spec.rb` have been ported.
* Some tests are currently skipped because they require additional setup.
  * [ ] Provide/import test data.
* Some tests are currently skipped because they don't match actual behaviour.
  * [ ] Fix tests, possibly using guard clauses (`required_version`).
* [ ] Add more tests, particularly investigating version-specific behaviour.
  * [ ] Test query continuation (#58).
* [ ] Add documentation (here and mediawiki-testwiki).

Result:

    Finished in 1 minute 37.12 seconds (files took 0.86261 seconds to load)
    139 examples, 0 failures, 25 pending