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.
What would be helpful here is a `Gen` for Argonaut JSON, which can be trivially converted to an `EphemeralStream` from which you can pull N elements, render as strings, etc.