timer: ref/unref return self

Most calls to ref() and unref() are chainable, timers should be
chainable, too.

Typical use:

    var to = setTimeout(ontimeout, 123).unref();

PR-URL: https://github.com/nodejs/node/pull/2905


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trevnorris@nodejs.org>