enh: make this module compatible with mongdb replica-set. fixes #2

Make this module compatible with mongdb replica-set:
- add import ReplicaSetConnection from pymongo
- add "replica_set" parameter in "get_instance" method
- add "self.replica_set" paramater in "__init__" method 
- making connection to mongodb replica-set if parameter provided in cfg
- finally edit and add "replica-set" parameter to "/etc/modules/mongodb.cfg"

Briefly, I reuse bulks of code from mod-retention-mongodb which already handles Replica-set connection.