Some refactoring

I added sesame-model dependency to start developing Sesame support (sesame model only contains Value/Resource/Literal/BNode classes). In Future it should probably moved to separate sub-project but as there is no code there yet I think it is ok to develop it here.
I've also made some small refactoring and code cleanup
* I deleted redundant dependencies and parentheses as well ass val declarations in case classes (that are also redundant as case classes generate vals inside)
* I created rdfgraph package and moved all RDF classes to make this consistent with current package structure (where we have qgraph package for quad graph classes, tgraph for triplet graph, I think rdfgraph package will fit nice with RDF classes)
* I spitted huge files into packages (to make it easier to work with them), so now there is rdfgraph.nodes package, rdfgraph.statements package (for rdftriplets and in future also for rdfquads). I also extracted Ground and Exists into separate files as RDFGraph.scala was too big to deal with it and also to fit current package structure (where we have graphImpl as separate file, as I understood Ground is actually RDFGraph implementation).