Misc(TEX): Minor typographical tweak to the latex template.
Misc(MD): Change the 'Proof (of #rf)' syntax to 'Proof (of {ref:#rf})'.

  Makes it more consistent.
Misc: Fix a typo in a doc string.
Feature(CORE): Add a get_child_by_css_selector method.
Misc(TEX): Escape more characters (&,%, ...) + change the logic of _render

  -- _render(node) now takes care of node.text + all child nodes
     (except for some, which are ignored) + their tails
  -- _render(node) does not take care of node.tail !
BugFix(TeX): Better handling of 'Proof (of ...)' type stuff.