fixed doctests.  added check for circular links.  added a test for Collection.append that was failing proper usage of rdf:nil. fixed append mechanism to properly accomodate rdf:nil.
fixed doctests.  added check for circular links.  added a test for Collection.append that was failing proper usage of rdf:nil. fixed append mechanism to properly accomodate rdf:nil.

RDFLib/rdflib,dbs/rdflib,RDFLib/rdflib,yingerj/rdflib,marma/rdflib,dbs/rdflib,armandobs14/rdflib,yingerj/rdflib,RDFLib/rdflib,dbs/rdflib,marma/rdflib,avorio/rdflib,ssssam/rdflib,ssssam/rdflib,yingerj/rdflib,marma/rdflib,dbs/rdflib,avorio/rdflib,ssssam/rdflib,armandobs14/rdflib,avorio/rdflib,avorio/rdflib,armandobs14/rdflib,RDFLib/rdflib,yingerj/rdflib,ssssam/rdflib,armandobs14/rdflib,marma/rdflib