Minimum changes to use knitr as vignette engine for Rrst vignettes

* I added the minimal changes to allow knitr to be a vignette engine for Rrst vignettes. 
* R CMD check knitr_* doesn't give new warnings and I used it successfully to compile a Rrst vignette via R CMD build.  
* Maybe we should also update knit2pdf to have a ``quiet`` argument for consistency?