Changing the link to grunt.js installation instructions.

Based on user feedback, it was very non-obvious that if using an
web server other than multiplex/index.js it would be necessary to
upload the socket.io client libs. This negates that problem.

Essentially, slides have variable height now. This is not normally visible, but with transitions that have a non-transparent background, like `cube` or `page`, it is:




http://lab.hakim.se/reveal-js/?transition=cube




Before, slides had a fixed height, which means that presentations designed with that assumption in mind will look very different now.




Another aspect that I bumped into is that I used the slide's height to dynamically size images to fill the remainder of the space from the heading to the footer, like this (stripped to the essence):




          bigImage.style.height = section.offsetHeight - bigImage.offsetTop + 'px';




Alternatively, is there an easy way to restore the previous functionality?

Thanks for the multi-control support!

fix vertical transition for vertically centered slides, vertical centering is now on by default (#70)