Add R

Adds R as distributed by the R project. It uses the rstudio.org mirror, because that one uses a CDN, as opposed to the project's main server, r-project.org. See http://cran.r-project.org/mirrors.html for details.

Note: the stanza for uninstalling the cask almost works, but leaves behind a few files that aren't in the 'pkg' manifest and appear to be a cache created when the package was run. For example:

/Library/Frameworks/R.Framework/Versions/3.0/Resources/fontconfig/cache/CACHEDIR.TAG

If you know how these could be (safely) detected and removed, please let me know. Even so, the package can be re-installed after uninstalling like that, and everything still seems to work fine.