async: use three args for addEventListener

The third argument for `addEventListener` and `removeEventListener`
is supposed to be optional, defaulting to `false`, but some browsers
(notably older versions of FireFox) require it.

See https://github.com/thinkpixellab/PxLoader/issues/5 and
https://developer.mozilla.org/en-US/docs/Web/API/EventTarget.removeEventListener
Merge pull request #96 from jamesarosen/james/addEventListener

async: use three args for addEventListener