node: implement unhandled rejection tracking

Implement unhandled rejection tracking for promises as
specified in https://gist.github.com/benjamingr/0237932cee84712951a2

Fixes #256
PR-URL: https://github.com/iojs/io.js/pull/758


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Domenic Denicola <domenic@domenicdenicola.com>