Javadoc to RDF-1.1 Concepts and Abstract Syntax

Adds javadoc linking to the relevant parts of the RDF-1.1 Concepts and Abstract Syntax specification.

Removes Resource per previous discussions about conflicts with the RDF-1.1 Concepts and Abstract Syntax specification, and replace with BlankNodeOrIRI, pending further discussion about a possibly more suitable interface name.

Replace getProperty with getPredicate per RDF-1.1 terminology for the Subject, Predicate, Object positions in a Triple.

Also add java-8 to the pom.xml, so it is recognised by Eclipse as a Java-8 project.

Add the Apache License to pom.xml to programmatically advertise the license.