Add json_file output format (uses json-c)

Example output for syn-scan:

{ "type": "result", "response-type": "rst", "saddr": "10.222.49.19", "daddr": "192.168.0.4", "proto": "tcp", "sport": 80, "dport": 59284, "seq": 0, "ack": 3135547661, "in_cooldown": 0, "is_repeat": 0, "t": "2013-08-17 14:01:08", "ts": 1376766068, "tm": 472 }