Add MongoDB Support

[Jaydar]

What we need more is that MongoDB, DynamoDB and Cassandra have fewer users compared to the former.

[carllerche]

MongoDB support should be relatively easy to add. DynamoDB/Cassandra were mostly picked as the first targets because of the limited feature set (if it works w/ those, it should work w/ most). There are a few significant driver related re-factors in progress, but after that toasty should be in a good place to add more DB support.