add @fhirschmann to the contributor list, and a news item for #789; also re-roxygenize the doc
just use stderr=TRUE for system2(), and see if we need to stop according to the chunk option `error` and error status

#789
smaller package vignettes by not embedding Bootstrap and highlight.js
add the `envir` argument to knit_child(), per request of http://stackoverflow.com/q/24009622/559676
tweak the doc of `envir` in knit()
add a note on reproducibility and purl()
let apt-get and R be quiet if possible
fixes #788: increment in the chunk counter even if chunks were read through read_chunk()
fixes #790: chunk reuse via <<label>> can fail when the first line of the reused chunk is empty
bump version