codec.d was missing the function declarations and the loader for the libtheora shared library. Oops!
Added theora.d. At some point, will add a package.d, but not until I'm ready to do it for all the Derelict packages.
Updated the README to reflect the new additions.
Changed some types in codec.d to use the ogg types as declared in the C headers since I need to import derelict.ogg.ogg anyway.
One of the DLL names in both theoraenc.d and theoradec.d was wrong.