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.
This code executes an effect outside of `Task.delay`, so it's impure. Instead, consider:

```scala
val genTempName: Task[Path] = Task.delay {
  Path( "gen_" + scala.util.Random.nextInt().toHexString)
}
```

The id is not actually used in the above snippet, so it won't change uniqueness, but if you are worried you could always use a GUID as part of the file name.