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.
Yes, I do feel generating different documents would help, primarily because there are some places in the code that can belowup depending on document size. e.g. on crunchbase, their API returns some JSON for companies which can be multi-megabyte (or more). With fixed chunk size, this will likely blow the process. Nonetheless, we can open a separate ticket for that.

Is this already for me to take another look at?