added stardog as vivo backend

It won't let me create a pull request for a new branch, so you'll have to do that on your end.

To get it to run, you'll need to get a stardog license.  You can do this for free, and I put a little info about it in the README.

The .gitattributes has a clause that tries to treat everything as text which was screwing up the binary files, but i eventually figured that out.

Let me know if you have any issues with it.