IE 8 support for <style> in passages

* Fixed a bug where IE 8 would turn <style> elements in passages into
<div>s.
* The exitprompt StorySetting is now off by default in the engine as
well as the IDE.
* Responsive: Fixed the bug where a blank rectangle would remain if the
bookmark link was disabled in StorySettings.
* Added an Array.prototype.forEach() polyfill to engine.js, as a number
of custom scripts seem to use it.