create subclasses for maildir and rdiff-backup. If you are using these features of backupninja, you'll want to switch to doing "include backupninja::client::maildir" for the machines that need it and this will pull in backupninja::client, so you don't need to include that anymore if you were explicitly doing so for those machines (although it shouldn't hurt). Right now what using these new subclasses will get you is dependencies on needed packages, but in the future there might be more.