test code for r_install
vectorized RInstall
apt-get update only once should be enough
do not install littler for now (for lack of /usr/bin/r on OS X)
make devtools installation optional yet automatic for the two functions using devtools
Merge pull request #30 from eddelbuettel/master

do not install littler for now and make devtools optional
Get pedantic about variable names.
Merge pull request #28 from krlmlr/nitpick

Nitpicking
Trivial cleanups after Kirill's fixes.
Re-add Kirill's cleverness with testing.