Consume existing mongo.Db, typo fix and some jsHint fixes

First of all i will thank you to get time and write this package for community. 

I was using this package in one of my app, but i wanted to use existing mongo connection(with replicaset settings), but not found the way to do it with current version, so i added in my stuff to handle existing mongo.Db instance. Also i fixed 2 of the typos and did some jsHint cleaning.

I hope these changes can help improve the current version.

Excuse me if that makes no senses to package.

Thanks