Add mongodb support to gutterball

Testing is still a bit of a PITA at the moment, and require a live DB -- ugh. Played with Fongo, but there are some features of the mongodb-java-driver that are not currently supported.

TODO: Find a way to isolate these tests as integration tests.

NOTE: Some tests currently require a mongodb setup in order for them to be run.
> Is it proper to expose DBCursor? Seems like this would be a List<xxx>

In my mind, no. But the DBCursor will lazily fetch the records as it iterates. If we convert to a list, we get them all at once. If the collection is large we'd be pulling a lot. We could perhaps wrap it in a typed Iterator.  Iterator<M>

I'll try that out to see how it goes.