Converted vignette to use knitr rather than sweave. Knitr's big advantage is its automatic caching, which makes rebuilding the vignette much faster.
For info on using knitr for vignettes see http://yihui.name/knitr/demo/vignette/
The conversion from sweave to knitr was done with Sweave2knitr, see http://yihui.name/knitr/demo/sweave/