Allow Operator to Configure Redis

Depends on #14

Caching backends are kind of a pain to configure in Drupal. You have to modify settings.php and you have to load a file from a module without the module being enabled because caches get loaded before modules. Ideally, there would be a notion of library dependencies that actually mattered, and then the redis backend would just be in a vendor dir and automagically added to an autoloader. Keep dreaming.

Instead, we now have a notion of a drupal_drush_command resource, which is used by a shared_module resource to download so-called modules such as these to a place where the server can just share them among all sites. This way, we don't have to worry about the actual project repository having the backends that it ends up needing and having available in an actual given environment. So, in theory, if a simoon blows away all the redis servers, you could configure the application to connect to memcached .