Don't track PDF version of slides
Remove ridiculous hack needed to get S4 to work in slidify

There was a bug in slidify that prevented using setClass and
setMethod inside a slidify presentation. Ramnath fixed this in the
dev branch of slidify.

However, a separate problem has arisen which is that the current
presentation does not compile correctly with the JHSPH template.
Needs investigation....
Merge pull request #11 from rdpeng/classes-methods

Classes methods