Remove smaller digits as defaults

A smaller digits value causes some problems with printing results correctly. See #768 for more details.
the `render` function must have at least 2 arguments, thanks to @kohske
and normal_print() must have two arguments as well
fixes #779: when tidy=FALSE and eval=FALSE, prompt=TRUE should still work
bump version
factor out the code to check if tcltk is available and if I'm in the R CMD check process (may not be accurate, but I guess it should work)
cosmetic
fixes #784: also provide vignette engines with tangle disabled
roxygenize
bump version
redirect stderr to code chunk output
add code chunk option to disable redirection of the engine's stderr
consistent parentheses
update reference to system
Merge pull request #789 from fhirschmann/engine_stderr

Redirect an engine's stderr to its stdout
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()