Added phantomjs script for easier printing

Added `js/print-pdf.js` so you just need to run the following command on the console to have your reveal slides printed to a pdf:
`$ phantomjs js/print-pdf.js [reveal-file.html] [output-file.pdf]`

(requires phantomjs, of course)

// edit:
default filenames are `index.html?print-pdf` for input file and `slides.pdf` for output file