Adding Identifiable functionality

Allows use of arbitrary ActiveModel-like models in ActiveTriples.
Refactoring Identifiable to remove ObjectResource

A special class is unneeded, Class.new(AT::Resource) works fine.
Also removed duplicate property delegation.
More thorough constant cleanup in Identifiable spec
Replace accessors with ActiveModel hooks
Allow #update_resource to take a block
Remove #write_attribute, delegate #property
Move block yield within property loop
Merge branch 'feature/identifiable'

Conflicts:
	lib/active_triples.rb