siblings = function siblings(container, mount, cb) {
	  mount = [].concat(mount);

	  [].forEach.call(container.children, function (node) {
	    if (mount.indexOf(node) === -1 && isHidable(node)) {
	      cb(node);
	    }
	  });
	}
Firefox doesn't have a focusin event so using capture is easiest way to get bubbling
IE8 can't do addEventListener, but does have onfocusin, so we use that in ie8

We only allow one Listener at a time to avoid stack overflows