Install an R kernel by default!

Installs an R kernel, as well as a few extra packages:

* ggplot2
* Hmisc
* plyr
* randomForest
* RColorBrewer
* reshape
* reshape2
* stringr
* XML

The installed packages came from me looking at top packages and knowing which I've used in my old decaying mathematical past.

/cc @takluyver for his R background and, well, he wrote the R kernel!