Fix up cross-browser support

Ok, cleaned things up a bit. Instead of the kludgy "save original URL" workaround, I switched the platform.js link to use RawGit for the latest build. When they cut a new release, it can be pointed back to cdnjs.

I also noticed a couple other bugs and cross-browser issues in the "stoppable" and "audio" sample pages. Cleaned those up in distinct commits, but it seemed like overkill to submit them in a separate PR.

This PR is running on my gh-pages instance: http://willi.am/x-gif/