Move unpacking functionality into a separate Util::Unpack class

Patch contents:
 * Better modularity for unpacking of input Zip archives
 * Small bug fixes with unpacking (guessing the main TeX file)
 * A bit of extra documentation for LaTeXML.pm
 * New enhancement: when we have an anonymous archive-to-archive conversion (i.e. without a destination), a placeholder destination is provided for the specific conversion job.

In arXMLiv we proivde a literal:zipbinary and expect a literal zip binary on output, without any extra information for the source TeX file, or the destination HTML file. The guessing of the TeX file I did last week, this patch also includes the follow-up guess of the HTML destination, for full automation. I am going to use this extensively for arXiv.

Also, even though the pre-commit changes are already adopted by you and rebased back by me, they appear in the pull request contents, which is confusing... But shouldn't cause problems upon merging I think. I will refactor Post::Pack to Util a bit later, since I don't want to get too distracted from my arXMLiv work at the moment.