Support RD

About RD cited from http://uwabami.github.com/rdtool/:

    RD is Ruby's POD. RDtool is formatter for RD.

    RD is multipurpose documentation format created for documentating
    Ruby and output of Ruby world. You can embed RD into Ruby
    script. And RD have neat syntax which help you to read document in
    Ruby script. On the other hand, RD have a feature for class
    reference.