Factors out behavior from ActiveFedora::Rdf::ObjectResource to mixin
ActiveFedora::Rdf::Persistence.

The change allows any ActiveTriples::Resource descendant which includes the mixin
to be used as the resource class for an ActiveFedora::RDFDatastream, rather than
requiring an ObjectResource subclass.
Remove ObjectResource from RdfDatastream

Introduces a better interface for using existing Resources to back a
datastream. Removes ObjectResource and related singleton_class calls.

Pins to ActiveTriples 0.4.0, which restricts property names for
Resources & RdfDatastreams to those not already used by a method on the
class (prevents overriding `class`, `graph`, `repository`, `name`,
etc...).
Merge pull request #500 from no-reply/object-resource

Object resource.
Merged into 8.0.0.pre1 as publicized on hydra-tech.