Blanknode identifier only

[stain]

A variant proposal of #63

I putting this patch as a proposal for discussion, not giving my +1 yet.

tl;dr: BlankNode.identifier() returns a "globally unique identifier" - recommends UUID or Skolem IRI. Used for equality checks.

Added:

The identifier MUST be globally unique. It is RECOMMENDED for the identifier to be a UUID string. If the identifier is a IRI, it is SHOULD be a skolem IRI.

It is NOT RECOMMENDED for this identifier to form part of the RDFTerm.ntriplesString().

Javadoc: http://stain.github.io/commons-rdf/blanknode-identifier-only/

Same discussion as in #63 applies, except:

• Implementations are more at freedom for what kind of identifier scheme they use
  • BUT: Must make those globally unique. "jena:graph5:b4" won't do. :)
• Users of the API might be more likely to mess up by sending in b1 as the string to the RDFTermFactory.createBlankNode(string). Javadoc warning added.

[coveralls]

Coverage decreased (-3.81%) to 80.11% when pulling b350c17 on stain:blanknode-identifier-only into 61f97fc on commons-rdf:master.