Fix TOC generation to match older Maruku.

Maruku 0.6.1 would notice if a single header encompassed the entire document, and would make that the root section of the document, as well as setting the title to that header's content. This restores that functionality.
Remove Nokogiri dependency from the S5 slides output.

This change switches to using the HTML output utilities to generate S5 slides. It also restores the STDOUT printing of slide titles
that current Maruku does, but only from the CLI (it won't annoy users who programmatically generate slides).