add a note about jvmr
a note on runr
fixes #851: coerce any objects that are not data frames to data frames

e.g. for dplyr objects, x[, j] later can be interpreted as x[, 'j'], which is wrong, even though x is also data frame
when using Andre's highlight, I need to make sure \hlesc, \hlpps, \hllin, \hlslc are defined
the warnings about missing Pandoc/rmarkdown should be delayed till weaving time: it does not make sense for them to show up at package loading time
bump version