make mtc time a string

Was a timestamp, but [RFC 4672](http://tools.ietf.org/html/rfc4627.html) doesn't specify a datetime type. Python `json` didn't like it:

```python
{
"type": "mtc_status",
"GTID": 5814971,
"cnt_10MHz": 5421765238271040,
"time_10MHz": 2013-03-07T04:35:23.83Z,
"data_available": false,
"read_pointer": 25344,
"write_next_pointer": 25344,
"num_records_in_mem": 0
}
Traceback (most recent call last):
  File "pmtwatch.py", line 32, in <module>
    oj.run()
  File "pmtwatch.py", line 22, in run
    print json.loads(msg)
  File "/usr/lib/python2.7/json/__init__.py", line 326, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 382, in raw_decode
    obj, end = self.scan_once(s, idx)
ValueError: Expecting , delimiter: line 5 column 19 (char 90)
```