removed notion of default context; added load and remove_context
removed notion of default context; added load and remove_context

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