show polymorphic project owner in create response

closes #310.

This PR depends on a PR for our version of RestPack::Serializer https://github.com/edpaget/restpack_serializer/pull/2 - **DON'T** merge this PR till those changes are in Ed's dev branch.

@edpaget - I haven't issued any PR's for the changes to RestPack::Serializer, any chance you could issue them?