Connecting to other databases #38

We mostly tested Graph2VR with Virtuoso servers like DBpedia. That worked well, but we could not get it to work with, e.g. GraphDB or Allegro Graph yet; even so, they would be supported by DotNetRDF.
Under the hood, different functions in DotNetRDF need to be called when connecting to those databases.
We haven't implemented that yet, but it would be great to connect to such kinds of databases, too.