Refactor tests

I’ve completely refactored rspec tests to eliminate a huge code duplication, improve tests structure and provide a more descriptive assert messages (using a custom [rspec matcher](https://github.com/jirutka/asciidoctor-htmlbook/blob/701dcb916b039fbc3928309c607eb8ce00679189/spec/support/matchers/have_html.rb)).

There’re some tests that fails in my environment, but these are failing even on master branch, it’s probably due to [this require](https://github.com/jirutka/asciidoctor-htmlbook/blob/master/spec/spec_helper.rb#L3) (I don’t now where to get it).

I’ve updated rspec to the last version (to use some new features), so you must run `bundle install` before running tests.

Note: This PR depends on #46!