Add live tests to verify actual API usage.

Requires Docker and mediawiki-testwiki. Images are available at 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
Merge pull request #68 from blackwinter/add-live-tests

Add live tests
Welcome Jens Wille
Version bump (0.6.2).
Cleaned up `require`s.

* Use `require_relative` where applicable.
* `require 'rake'` not needed in Rakefile.
* `require 'rubygems'` not needed with Ruby >= 1.9.3.
* Drop Rails 2.3.x/RubyGems incompatibility workaround. (Introduced in 302097a; cf. http://stackoverflow.com/questions/5176782)
Provide entry point matching gem name.

Just as a courtesy for newcomers.
Move version constant into its own file.

No need to load all the dependencies to get at the version number.
Specify required Ruby version.

Settling for 1.9.3 as anything older should really be dead by now.
Made MediaWiki::Gateway#custom_query public.

That was most probably an oversight in 1350fc4 (PR #38 by @shouya).
Formatting cleanup and syntax update.
Specify license in GemSpec.