nsq_to_file: Ability to set time

Hi everyone,

I love nsq. It's powerful, easy to setup and has great tools included. The problem I have right now is nsq_to_file doesn't include the time of message reception. 

I know the written files are format agnostic, but I guess a lot of people use text format and having the ability to have a timestamp before each written line might be of interest to some of you.

### Result

This allows to have something like that:

NSQD: 

    2014/08/24 08:16:31 opening /tmp/events.xps.2014-08-24_08.log

LOG FILE:

    $ tail -f /tmp/events.xps.2014-08-24_08.log
    17-52.184 | {"call":"device_connected","connection_id":"42","from":"receivers/xps","source":"127.0.0.1:57540","to":"events"}
    17-53.129 | {"call":"device_identified","connection_id":"42","device_id":"f042aa92-b965-80ab-a832-6d4b878f3d33","from":"receivers/xps","source":"127.0.0.1:57540","to":"events"}
    17-55.130 | {"call":"device_disconnected","connection_duration":"2","connection_id":"42","device_id":"f042aa92-b965-80ab-a832-6d4b878f3d33","from":"receivers/xps","source":"127.0.0.1:57540","to":"events"}


Could you please consider merging this into the `nsq_to_file` app or providing a similar functionality.