Fix Firefox 24 issue related to FocusEvent

FF24 does not support `document.createEvent('FocusEvent')` and to create a new
FocusEvent we need to use `new FocusEvent('x')`.

IE on the other hand does not support event constructors so we have to branch
here.
Merge pull request #174 from arv/firefox-24-focus-event

Fix Firefox 24 issue related to FocusEvent