Update Changelog
Add Hexp::ROOT, Hexp::Node#tag?
Update RSpec syntax to 3.0
Starting point for an HTML unparser (generator)

Before we were turning a node into a Nokogiri DOM, then letting
nokogiri (libxml2) do the generating for us. This, however, is horribly
slow, and also somewhat unflexible. e.g. libxml2 the last time I checked
did not have a sense of what HTML5 is. It's either XML, or "html"
meaning something HTML4-ish. This pertains to escaping rules,
self-closing tags, doctype, etc.

This implementation is minimal for now but should already be good enough
for most needs. The previous commit contains a few extra tests to smoke
test the escaping.