added an asymptote engine to knitr
added example Rmd file for asympote engine
removed example (needs to go to different repo), and merge dot engine and asy engine.
added myself as a contributor
Merge pull request #559 from tlamadon/master

added an asymptote engine to knitr to generate nice graphics
use `# ----` instead of the narcissistic token `# @knitr` for purl() and read_chunk()
tweak comments for eng_dot()
cosmetic
write a message before running the command
fig has been created before, no need to do it again
news for #559
generate the temp file under the current directory, just to make the command shorter
strip white spaces
# ---- instead of # @knitr
there is no \SweaveOpts{} now
make examples (lyx 2.0.6)
roxygenize
bump version
do not include code chunk when purl=FALSE
expand a few chunk options for purl()

per suggestion of Thell http://stackoverflow.com/questions/16431100/knitr-exclude-chunks-from-purl/17140654?noredirect=1#comment24941826_16431100