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.
Consider triple quotes so you don't have to escape double quotes or other chars, or building the JSON AST and rendering it to JSON to prevent malformation errors.