Add Gemfile that points to gemspec

Also add required Ruby and gems' versions to the gemspec file (as specified in README.md).

Note that Nokogiri 1.6 can't be used with Ruby versions older than 1.9.2 so the upper version is limited to 1.6 (or Ruby version requirement needs to be bumped up).
Merge pull request #4 from sidonath/add-gemfile

Add Gemfile that points to gemspec