Added new r-package build-type.

Added a new ```r-package``` build type so that R packages can be specified in the config file (before they were listed within ```lib.custom```).

A separate but related issue has also been fixed where R package installation failures were silent. We will now see if/when a package fails to install.

A corresponding pull request will be issued to the qiime-deploy-conf repo with the necessary changes.

This has been tested on an Ubuntu EC2 instance. Fixes #3 and #15.