timers: improve Timer.now() performance

Record the start time so we can make the return value of Timer.now()
relative to it, increasing the chances that it fits in a tagged integer
instead of a heap-allocated double, at least for the first one or two
billion milliseconds.

PR-URL: https://github.com/nodejs/io.js/pull/2256


Reviewed-By: Trevor Norris <trev.norris@gmail.com>