added set methods or, and and xor for graphs, as suggested by issue 119 Fixes issue 119
added set methods or, and and xor for graphs, as suggested by issue 119
Fixes issue 119

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