ignore readme.md in R CMD build
add date
if stop_on_error == 2, the error must have been emitted in evaluate_call(), so it is impossible to come here
added keep_warning/keep_message in evaluate() so that we can discard warnings/messages when desired

related to yihui/knitr#571
when stop_on_error == 2, there is no need to set up an error handler
roxygenize
bump version