Rdflib-rif

Implemented support for lang in rif presentation Syntax
Implemented horn-Syntax as presentation for RIF.Implies, ex:

Forall ?x ( ex:a(?x) :- ex:a(?x) )

Non-RIF/XML documents will now be rejected if an unsupported predicate is identified.

This first release provides a parser for rif documents and rif-presentation document for rdflib

After installation rdflib is able to parse rif/xml per:

rdflib.Graph().parse("path/to/rif.rif", format="rif")
rdflib.Graph().parse("path/to/rif.rif", format="rif/xml")


You can also parse the [presentation syntax for the production rules](https://www.w3.org/TR/2013/REC-rif-prd-20130205/#sec-presentation-syntax):

rdflib.Graph().parse("path/to/rif.rif", format="rifps")
rdflib.Graph().parse("path/to/rif.rif", format="RIFPRD-PS")

The presentation language for BLD is currently not supported.