markdown is a required package now, so no need to use if (require('markdown'))
800px for the R HTML vignette
roxygenize
.External2 is for R 3.0 only
define isR3 and some known empty calls
redefine plot_calls() under R 3.0; it is straightforward to tell if a plot is empty under R 3.0

this fixes http://stackoverflow.com/q/16051436/559676
simplify comments
should be careful with palette() as well under R 3.0
bump version