* Added replicaset name to bigchaindb config
* changed travis replSet to match bigchaindb default
* Updated initialize_replica_set
It now initializes the replica set with the name provided by the
bigchaindb config file.
* initialize_replica_set is now called when creating a mongodb connection.
This way we are sure that a replica set exists when we return a
connection.
* Moved the initialize replica set logic to the connection.
* update the config documentation
