No longer subclass Nokogiri::XML::Node

WIP: Do not merge yet.

Subclassing Nokogiri::XML::Node is off the beaten track of Nokogiri usage, and causes all sorts of pain, especially on JRuby. It's also unnecessarily complicated and results in a lot of copying of data.

The solution is to use delegation more, and adopt the Nokogiri way of doing things better. This branch is moving in that direction, starting with the elimination of Niceogiri.