Refactoring ActiveFedora::Rdf to use ActiveTriples

This moves the general functionality of ActiveFedora::Rdf to the
external ActiveTriples gem. Release 0.2.0 of that gem is up-to-date
with the latest pull requests on this codebase, and adds ActiveModel
and JSON-LD support. Development will continue there in partnership
with Linked Data for Libraries and other interested communities.

This supersedes #446.

Please review to ensure that everything is okay and that I've cleaned 
up what needs cleaning up.
Okay, really done now. I think this PR is ready to go.