ofctl_v1_3: support OFPMeterConfigStats

this patch implements a REST API for OFPMeterConfigStats.

e.g., when you send a MeterMod message as:

    curl -X POST -d '{"dpid": 1,
                      "meter_id": 1,
                      "flags": "KBPS",
                      "bands": [{"type": "DROP", "rate": 1000},
                                {"type": "REMARK", "rate": 2000},
                                {"type": "EXPERIMENTER", "rate": 3000}
                      ]}' http://localhost:8080/stats/meterentry/add

and you do the following command:

    curl http://localhost:8080/stats/meterconfig/1

then you will get the configuration as:

    {"1": [{"bands": [{"burst_size": 0, "rate": 1000, "type": "DROP"},
                      {"prec_level": 0, "burst_size": 0, "rate": 2000, "type": "REMARK"},
                      {"burst_size": 0, "rate": 3000, "experimenter": 0, "type": "EXPERIMENTER"}],
            "flags": "KBPS",
            "meter_id": 1}]}