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.
That's a good idea, but more than I needed for this test, unless you think generating more interestingly different documents is important. I just wanted to force the driver to send lots of bytes over the network, and this seems to do that.