Improve handling of old versions of Firefox (issue #6)

Issue #6 points out two problems with old versions of Firefox.

1. <code>link.parentElement</code> has been changed to <code>link.parentNode</code>. This makes things work in Firefox 8.

2. @apei830 also pointed out an issue with <code>insertAdjacentHTML</code> (not supported by Firefox 7 and under). I don't think it's worth trying to support those versions (they're not mobile anyway). Instead, I added detection for <code>insertAdjacentHTML</code> so the user does not get an error.

In summary, the changes expand support to Firefox 8 and get rid of errors for Firefox 7 and under. I did a quick spin on BrowserStack with old versions of FF, iOS, Android, Opera Moblie and the modern desktop browsers to make sure these changes don't cause other issues.