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.

Conflicts:
	lib/active_fedora/rdf.rb
	lib/active_fedora/rdf/object_resource.rb
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 #535 from projecthydra/at0.4.0

At0.4.0