Fix requires and directory/module layout

Here is the third round. I still didn’t have time to write a rationale for all the changes, so I’m sending just pull request and will comment it later (hopefully in 1-2 days).

In the meanwhile, please read the updated usage instructions in readme. tl;dr: use `./bin/asciidoctor-latex foo.adoc`.

Please not that GitHub doesn’t follow renamed files in diff, so it’s better to review it in a classic way using `git diff`.