when caching the package names, exclude the base packages, so don't need to do it when loading packages

note setdiff() does unique() automatically
use writeLines() instead of cat(sep = '\n')
sort the package names before saving them to __packages
Frank's name should be 'Harrell, Jr, Frank E'
add @BrianDiggs for #489
knitr does not really need RTools; Windows users can install from source on RForge
duplicated -> duplicate
add Jeremy Stephens to contributors for #490
roxygenize
cosmetic (the envir argument has been added in #490, so close #491)
bump version