Adding attributes and json serializer to Resource

Adding attributes method to resource. This supports
ActiveModel::Serialization. This is a first step toward JSON-LD support
with the pragmatic_contexts gem.
Adding support for unregistered attributes

ActiveModel style #attributes now return URIs and vales for predicates
without a registered property name.
Adding unmodeled URI/Resource attributes
Adding better tests for Resources in attributes
Adding non-model predicates to serializable_hash
Removing unnecessary require
Merge branch 'feature/attributes'