Feature/issue 22

Bugfix for #22 described by v4hn. Adds duck-typed parsing for <transmission> tags so that either the new transmission types can be parse or the old PR2 types can be parsed transparently.

Added in `calvin` robot description, along with test parsing and generated patch, as well as a test parsing both transmission types in the same robot.

(After reviewing this old code, I realized that I really need to document this and clean it up so it's a little more understandable!)