integration tests for the MongoDB FileSystem (see #104)

This is just a start, but provides some coverage for a couple of upcoming tickets. This setup supports writing specs2 tests that require a backend and should not run in Travis.

Try `./sbt it:test` to see it in action. As is, it connects to a local mongodb instance, but you can edit one line to run against mongolabs.
Yeah, that's what I had and then at some point I decided I wanted to be give all the files a common prefix so I could clean them up more precisely, but I'm not doing that yet anyway, so I might as well do the simpler thing.