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.
Add tests for XML results parsing.
Simplified xml tests slightly.
Update node hash when parsing json.
Add tests for json parsing.
Add csv parsing test.
Minor fixes to TSV parsing.

- save blank nodes in nodes table.
- Don't crash when columns are empty.
Add test for TSV parsing.
Make nokogiri really be a soft dependency and fallback to REXML if it doesn't load.
Finish nokogiri. This addresses issue #39.
Merge branch 'origin/master'
add read_timeout support for requests
Test both with and without nokogiri.
Version 1.1.1.
Finish 1.1.1