New option when emiting JSON to add an ISO 8601 timestamp, bug fix for JSON+time filtering options

Two patches to code:
1. Bug fix for crash when combining --json with --before/--after options:
```
./dnsdb_query.py --before 1358051711 --after 1358051709 --json -r cisco.com/A
{"count": 79286, "time_first": 1358051710, "rrtype": "A", "rrname": "cisco.com.", "bailiwick": "cisco.com.", "rdata": ["72.163.4.161"], "time_last": 1394039207}
```

2. New option to add an ISO 8601 timestamp (set to UTC) to the output when emitting JSON:
```
./dnsdb_query.py --human_time --json -r cisco.com/A
{"count": 79286, "time_first": 1358051710, "time_first_iso8601": "2013-01-13T04:35:10Z", "rrtype": "A", "rrname": "cisco.com.", "bailiwick": "cisco.com.", "rdata": ["72.163.4.161"], "time_last": 1394039207, "time_last_iso8601": "2014-03-05T17:06:47Z"}
{"count": 1, "time_first": 1331587834, "time_first_iso8601": "2012-03-12T21:30:34Z", "rrtype": "A", "rrname": "cisco.com.", "bailiwick": "cisco.com.", "rdata": ["172.19.25.68", "198.133.219.25"], "time_last": 1331587834, "time_last_iso8601": "2012-03-12T21:30:34Z"}
{"count": 150120, "time_first": 1277354894, "time_first_iso8601": "2010-06-24T04:48:14Z", "rrtype": "A", "rrname": "cisco.com.", "bailiwick": "cisco.com.", "rdata": ["198.133.219.25"], "time_last": 1358050608, "time_last_iso8601": "2013-01-13T04:16:48Z"}
 ```