use weak timeout (rather than strong timeout)

[coolaj86]

As per https://nodejs.org/api/timers.html#timers_timeout_unref, calling .unref() will mark the timer as weak in the event loop and then return the timeout object.

Fixes #819.

[fent]

cool! i wasn't aware of this API on timeouts, pretty handy.

one request, do you mind removing the process.exit() line from example/ffmpeg.js. it's only there because of the timeouts hanging.

[coolaj86]

@fent Done.

[fent]

thanks!