Fix a couple of 'CouchDB' references left in the MongoDB sink.
Add a method that allows the MongoDB Sink to be configured with an existing Mongodatabase instance.

This change allows a Mongodatabase to be used that is backed by a cluster of servers with failover and not just a single server on a single uri.
Merge pull request #250 from IanMercer/mongoDBSink

Allow the MongoDB sink to be configured with an existing MongoDatabase instance