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.
I'd like to match the resulting exception, except:
1. It's a mongo-specific exception, which is kind of against the intent of these tests
2. I would need beLeftDisj, which is not available because of the way these tests are set up in SBT

That comment's not worth much, so I'll delete it.