Web input enhancement

This pull request contains all web-specific enhancements of my LaTeXML fork, built around the LaTeXML::Util::WWW API. There is definitely place for improvement and I would be happy to discuss and continue working on this front.

The three classes that have core enhancements are:
 * LaTeXML::Util::WWW
 * LaTeXML::Mouth
 * LaTeXML::Package

LaTeXML::Converter and LaTeXML have overlapping enhancements from the pull request of the epub-foundation branch, relating to bookkeeping of the URLs given as input ( and sadly diffs due to spurious spacing )