Rename Resource to BlankNodeOrIRI. Fixes #9

There has been extensive discussion about what name should be given to the class of objects that are allowed in the subject position of an RDF Triple, when using standard (non-generalised) RDF-1.1.

Resource, although used currently in some libraries to represent this term is semantically incorrect, as BlankNodes are not solely 'resources', they can represent Literals also.

Other potential names including RDFSubject, Node, NonLiteral, Identifier, and Reference have had objections, including both semantic and stylistic objections.

For these reasons, I am proposing switching back to BlankNodeOrIRI to enable discussion about other important issues to continue.

The full discussion can be found at:

https://github.com/wikier/commons-rdf/issues/9