Fixed issue where extracted assets may or may not include the host in th...

I found that turbolinks didn't work on WebKit Nightly 8536.30.1, 538+. This was because the `extractTrackAssets()` function was using `node.src` and `node.href`, which returned either the full-URL complete with scheme and host or the path only, depending on whether we were looking at the current document or the doc created from the AJAX response. This causes `assetsChanged()` to always evaluate to true, even when the assets haven't really changed.

Using `node.getAttribute('src')` and `node.getAttribute('href')` instead of `node.src` and `node.href` ensures that  `extractTrackAssets()` will return strings exactly as they are in the HTML source, and make the  `assetsChanged()` more robust.