19/accept host as list in config

PR for #19 
A list of hosts may be provided, without stating the replicaSet they are part of. [It's advisable to have the replicaSet provided, but not mandatory](http://api.mongodb.org/python/current/api/pymongo/mongo_client.html#pymongo.mongo_client.MongoClient). Am I interpreting this wrong?
cc @claudiu-coman