Wrapped R's package system similarly to perlPackages

The R programming language has a package system and a package repository (CRAN).  This change adds support for R packages, modelled on the perlPackages expression(s).  I have split the R package expressions off into a seperate file r-packages.nix and populated it with a few packages.  It is not complete, there are many more packages on CRAN.  It may be possible to automatically populate these expressions.