prefer marked to pandoc for markdown2html

better consistency with live notebook
don't require mathjax on the page in mathjax utils

required for reusing mathjaxutils in node
allow utils.js to be loaded in node

where navigator is undefined
add marked.js entry point for md2html in nbconvert

now uses the exact same code as the live notebook
include marked.js in package_data
Merge pull request #4655 from minrk/marked-nbconvert

prefer marked to pandoc for markdown2html