A new plugin: footnotes

Hi,

I've implemented a footnotes & bibliography plugin for reveal.js, please consider pulling it in. An explanation of how it works is in plugins/footnotes/README.md

The idea is that sometimes you need to put fixed positioned footnotes but let their content change slide by slide, especially in scientific presentations. You also then need a slide with bibliography at the end. So I've implemented this. Only thing I am not sure how to resolve yet is a bibliography longer than a sensible count, let's say 10. Perhaps by enabling scrolling inside the slide, will need to think of it, but that's easy to style by oneself if needed. Please comment, I am willing to develop this further/fix it, as I will be using this really often.

Note: I've regenerated the .css files, this makes the diffs a little less readable, but if you skip the .css files which were generated, you will see a clear and readable changeset. I've also added a test index.html file to test/ to show how all of this works and sensible (I think) defaults to the original file, which does not generate bibliography but exemplifies how footnotes can be used.