Only attach 1 event listener for visibilitychange

[JustinTulloss]

Some browsers support multiple different names for the visibilitychange event, this patch ensures that only one event will be listened to. It prefers the specced 'visibilitychange', but falls back to prefixed versions.

Also adds support for the 'moz' prefixed event.

[isocroft]

@JustinTulloss can you fix the conflict on this PR so it gets a chance to be reviewed and included ?

Cheers!!