jsonp invalid object fix

If i use callback option with sample template
```push_stream_message_template "{\"id\":~id~,\"channel\":\"~channel~\",\"text\":\"~text~\", \"tag\":\"~tag~\", \"time\":\"~time~\", \"eventid\":\"~event-id~\"}";```

and get old messages i catch invalid javascript:
```callback1([{"id":1,"channel":"lupd2","text":"tt1", "tag":"1", "time":"Thu, 31 Jul 2014 00:36:03 GMT", "eventid":""},{"id":4,"channel":"lupd2","text":"tt1", "tag":"1", "time":"Thu, 31 Jul 2014 00:36:06 GMT", "eventid":""},]);```

last comma is needless.
p.s. sorry, maybe my fix ugly. I do not have experience in the development by c