add a warning about the missing rmarkdown package (rstudio/rmarkdown#243)
reorder arguments using roxygen2 4.0.2, thanks to @krlmlr
require scientific notations to be inside LaTeX math, e.g. you will have to write $`r 2e10`$ instead of `r 2e10`

#837 and rstudio/rmarkdown#160
add @ajdm for #838
use file_test('-d') instead of file.exists()
bump version