emit an event for the context changing from "suspended" to "running"

[jeremyml]

It would be great to emit an event for the context changing from "suspended" to "running". That way I can know if I can start playing sounds to the user on iOS.

Currently I simply monitor the same events _unlock() uses, but it would be cleaner to trigger from a SoundJS event.

document.addEventListener("mousedown", runAudio, true);
document.addEventListener("touchend", runAudio, true);