Add an RBinaryInstall command, and use it with c2d4u.

This patch does three main things:
 * add c2d4u to our apt configuration,
 * add a command to install a binary version of an R package from c2d4u, and
 * use c2d4u for devtools.

I also cleaned up one or two printed messages.

I think this will fix #20, #44, and #46. Any or all of @eddelbuettel, @krlmlr, and @hadley, could you take a look? (In particular, Hadley, could I ask you to run your current dplyr config against this?)