Merge pull request #2 from yihui/master

buidl R Markdown vignettes under R 3.0.0
0.71
compatibility with R < 3.0.0
no need to use LazyLoad (it has been obsolete for years)
R's own png() is good enough; no need to use Cairo; knitr needs to be in Imports according to R-exts in R-devel