CDAO: removing dependency on librdf

Users of the Bio.Phylo CDAO conversion modules have had problems installing its dependency, the Redland RDF library and its Python bindings, which can be a bit of a dependency nightmare. I've refactored the CDAO modules to use the pure Python module RDFlib instead, which is a simple "pip install rdflib" away. I've also added the ability to specify additional triples that should be applied to TUs, Edges, Nodes, and Trees.