notes plugin: open html file with relative path to the notes.js file

With this patch when notes.js opens the html page with the speaker notes, it'll find the correct path to it, relative to itself.

The trick is finding the `<script>` tag for `notes.js` and find the correct path from it's src attribute.

This is useful when reveal.js is included from another directory - in my case I have this structure:

    prez1/
    prez2/
    reveal.js/ (a git subtree)

including it with ../reveal.js/...