Use Nokogiri instead of REXML for parsing results.

Nokogiri is significantly faster, and now has a pure-java backend for
use on JRuby.

This patch potentially breaks backwards compatibility, as
parse_xml_bindings no longer accepts REXML elements.
Add Nokogiri dep to README.
Add Nokogiri as development dependency.
Merge pull request #36 from tomjnixon/nokogiri

Tom Nixon's Nokogiri changes.