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).
Interesting that this is allowed now, even though it is not alllowed in official XHR standards.