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.
Is this a good dependency restriction, or should we be more pessimistic? With ActiveTriples at 0.x.x, it may be a good idea to lock into >=0.2.0, < 0.3.0, since it's conceivable that API-breaking updates would be introduced upstream.

I could easily commit to bumping any such updates to 0.3.0; and we could always loosen the dependency in a future minor release of AF.

What is best practice, here? Thoughts?