JRuby support / Nokogiri

Hi, Thanks for the great work on this gem!

I'd love to use this gem, but I work in an area where we often deploy to JRuby. We can't use it because LibXML requires C-extensions, so that doesn't really work in JRuby. We actually used it in a hackathon and found out half-way through that we can only use it from MRI.

Would you guys be open to a pull request that converts the XML parsing to Nokogiri from LibXML? 

Nokogiri works great in JRuby (has a native Java fork) and has fewer dependencies in MRI than LibXML, so I think overall it'd be a useful thing for the gem.