added ruby-based generic API for calling DBus methods

I know it is little strange to have this file here, but it is kind of generic library call, so it makes some sense.

If the user wants to use it, he has to install additional packages. But still I think it is better than creating extra packaga (yast2-ruby-dbus?) containing this file only and requiring yast2-ruby-bindings and rubygem-ruby-dbus....