add Rscript to knitr engines: http://stackoverflow.com/q/15271406/559676
need to pass options$code to engine_output() because `code` might have been modified
signal a message before running the engine command
more compact code: use eng_interpreted() for eng_coffee(), eng_highlight() and eng_sas()
tweak eng_interpreted() for some special engines

- highlight and sas require the code to be in a file
- haskell and Rscript require multiple lines of code to be in multiple -e
- for other engines, they can accept the code in a single string
typo: should be options$code
awk, gawk and sed need the file argument to be in the last place
write R code in a file because currently Rscript does not accept blank lines in -e
leave a note for awk, gawk and sed; need to fix it in the future
no longer uses `svn rev` since R 3.0 alpha is out now
bump version