Add POST /replica_sets/<rs_id> for ReplicaSet commands.

This addresses #63 

Changes:
- Add method POST to `/replica_sets/<rs_id>` to make a command to a ReplicaSet. The REST interface and implementation are parallel to the way we do commands on Servers.

Example of awaiting replication after applying these changes:
    
    curl -XPOST http://localhost:8889/v1/replica_sets/myLaggyRepl -d'{"action": "await_replication"}'