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.
It works as far as I can tell. specs2 is playing a few games with the semantics of Results (you are required to return a Result, but the act of producing a Result throws if the matcher fails). Anyway it seems to do the right thing if you embed this in Task and then run it.