timers: cleanup interval handling

Uses `null` as the false-y value for `_repeat` as like other properties.
Removes un-reachable statement in setInterval’s `wrapper()`.

PR-URL: https://github.com/iojs/io.js/pull/1272


Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>