Create a helper to set the proper path and add type 

I don't love that `classes` is duplicated. What about `PyModule_GetName(m) + ".Vec3d"?
Also, or in addition, maybe we should make some helper to set the proper path and add the type without needing the cast.

_Originally posted by @llllllllll in https://github.com/quantopian/libpy/pull/161_