Switch to w3c-xmlhttprequest

Closes #790.
Brings several improvements over legacy node-xmlhttprequest, like
support for "responseType" and "response" properties (allowing binary xhr requests through 'arraybuffer' responseType, see spec); better event support, xhr streaming, and room for improvement (code is quite well structured).
Agreed. Porting driverdan's code should be easy. Is it worth it ? O mean, it's not like it was more secure...