Image preloading + Responsive changes

* Inserted engine code that causes external images to be preloaded, in
case people still want to use external images for whatever reason.
* Removed Responsive's Respond.js polyfill, because it doesn't work too
well with Twine's dynamically inserted stylesheets, and it's unlikely
most people will even use it.
* Changed Responsive's footer to a sticky footer, like God and all His
hideous bat-faced angels originally intended.