reimplementation of functions introduced since R-3.0.0

 Implementations of functions which have been introduced in
 R since version 3.0.0. The backports are conditionally exported which
 results in R resolving the function names to the version shipped with R (if
 available) and uses the implemented backports as fallback. This way package
 developers can make use of the new functions without worrying about the
 minimum required R version.