Add RDFLib TriG serializer

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https://raw.github.com/RDFLib/rdflib/04a019ecf5e86c5cf65bbabd6cbd31a787b
a24d6/rdflib/plugins/serializers/trig.py
Use modified RDFLib TriG serializer
Merge branch 'trig'
Merge branch 'master' of https://github.com/Data2Semantics/linkitup